I.  INTRODUCTION

    Table of Contents

    As required by the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act, the agency promulgated Federal Motor Vehicle Safety Standard (FMVSS) No. 138 Tire Pressure Monitoring System (TPMS), which required a TPMS to be installed in all passenger cars, multipurpose passenger vehicles, trucks and buses that have a Gross Vehicle Weight Rating of 4,536 kg (10,000) pounds or less, except those vehicles with dual wheels on an axle. Two alternatives were allowed in the final rule. [2] The first alternative required the TPMS to give the driver a warning when tire pressure is 25 percent or more below the placard pressure for one to four tires. The second alternative required the TPMS give the driver a warning when tire pressure is 30 percent or more below the placard pressure for any single tire. However, a court ruling [3] found that based upon the administrative record before the agency, only the first alternative was reasonable, and the court vacated the standard for further rulemaking consistent with its opinion. In response, this NPRM proposes to require that the TPMS give the driver a warning when tire pressure is 25 percent or more below the placard pressure for one to four tires.

    There are two basic types of TPMS in production, direct measurement systems that have a tire pressure sensor mounted in each wheel, and indirect measurement systems that determine tire inflation pressure by measuring relative rotational differences in the wheels.

    The indirect measurement systems are designed for use with the anti-lock brake system (ABS) and compare the relative wheel speed of one wheel to another. Wheel speed correlates to tire pressure since the rolling radius of a tire decreases slightly with decreasing tire inflation pressure. Since the current indirect measurement systems compare relative wheel speed, they cannot determine when all four tires lose air at about the same rate. [4]  Commenters to the docket indicated that current indirect measurement systems could not meet the proposal.

    This Preliminary Regulatory Impact Analysis has several new analyses as compared to the Final Economic Assessment. [5] The agency requests specific comments on these analyses. They are:

    1. Chapter V -- Property damage savings and travel delay savings from avoiding crashes.
    2. Chapter VI – Cost estimates for malfunction systems and malfunction warning lamps.
    3. Chapter VI -- Opportunity cost analyses to estimate the cost of the time it takes to refill tires and the cost charged for air to refill tires at some service stations.
    4. Chapter VII -- Net benefit analysis
    5. Chapter VII – Cost/Benefit analysis of the malfunction warning lamp
    6. Chapter X – Probabilistic Uncertainty Analysis



    II.  BACKGROUND and ALTERNATIVES

    Table of Contents

    The following section discussed the details of various types of Tire Pressure Monitoring Systems (TPMS), including systems currently in production as well as anticipated systems.

    There are two basic types of Tire Pressure Monitoring Systems (TPMS) currently available that can alert the driver while driving that the tire pressure is low:  direct measurement systems and indirect measurement systems. A direct measurement system measures tire pressure directly. A variation of the direct measurement system (a direct measurement system with a pump) will soon be available that can inflate the tire when it gets low, relieving the driver of that responsibility.

    An indirect measurement system measures wheel speed or something factors other than tire pressure. Most current ABS-based systems are indirect measurement systems. They measure wheel speed and then compare the variance in wheel speed from one wheel to another to determine whether a tire is under-inflated.

    Although not currently in production, we believe that it would be possible to produce hybrid TPMSs with performance characteristics of both direct and indirect TPMSs.


    Direct measurement systems

    Most direct measurement systems have pressure and temperature sensors in each tire, usually attached to the inflation valve. They broadcast their data to a central receiver, or in some cases to individual antennae that transmit the data to the control module, which analyzes them and sends appropriate signals to a display. This display can be as simple as a single telltale, or as complex as pressure and temperature displays for all four tires (or five if the spare is included).

    Direct measurement systems’ advantages include: (1) much greater sensitivity to small pressure losses, with claims ranging from +/- 0.1 psi to 1 psi; (2) the ability to directly measure pressure in any tire at any time, including before starting the vehicle, and including the spare tire. The disadvantages include: (1) the higher cost; (2) possible maintenance problems when tires are taken on and off the rim (sensors have been broken off). These systems have not been installed on many vehicles, although they have been used on cars with run-flat tires and as accessories on high-end luxury vehicles.


    Direct measurement system with a pump

    A direct measurement system with a pump has the same qualities as a pressure-sensor-based system, except that it also has the ability to pump the tire back up to the placard tire pressure. Each tire has a separate sensor and a pump. The system display is designed to give a warning when a particular tire needs to be continuously inflated and if the tire pressure gets too low, indicating that a particular tire has a problem and needs servicing. Unless there is a catastrophic failure or a rapid loss of pressure due to a nail or puncture, the pump can keep the tire inflated to get the vehicle to its destination. However, once the vehicle stops, the pump stops, and the tire may deflate. The advantages of these systems include: (1) driver convenience, (only need to worry about tire inflation when a warning of a continuing problem that the pump has to continue working to control); (2) better fuel economy, tread wear, and safety by keeping tires up to correct pressure. The disadvantages include: (1) the higher cost; (2) maintenance considerations - when rotating the tires, the pumps must stay on the same side of the car. These systems have not been installed on any light vehicles, although they have been used on a number of heavy trucks for several years. Because of cost issues, a direct measurement system with a pump has not been considered in further analyses.


    Indirect measurement systems

    The current indirect measurement systems utilize the wheel speed sensors of Anti-lock Brake Systems (ABS). They take information from the ABS wheel-speed sensors and look for small changes in wheel speed that occur when a tire loses pressure. Low pressure results in a smaller wheel radius, which increases the speed of that wheel relative to the others. The systems work by comparing the relative speed of one tire to the other tires on the same vehicle.

    The advantages for these systems include: (1) low cost [JW1] and (2) the need for only minor changes to the vehicle that has an ABS system, including a new dashboard telltale and upgraded software in the electrical system. Disadvantages include:  (1) not all vehicles have ABS, so costs are significantly higher for vehicles without ABS; (2) the indirect system cannot tell which tire is underinflated; (3) if all tires lose pressure evenly, it cannot detect it, since it works on the relative wheel speed; (4) in some current systems, some combinations of two tires being underinflated cannot be detected (e.g., two tires on the same axle or the same side of the vehicle). (Regarding #3 and 4, current ABS-based systems cannot detect certain conditions of low tire pressure. To meet the proposal, the ABS-based systems would need to be improved.)  (5) they cannot check the spare tire; (6) the vehicle must be moving; (7) they require significant time, sometimes hours, to calibrate the system and several minutes, sometimes tens of minutes, to detect a pressure loss; and (8) they cannot detect small pressure losses. (Regarding #8, the best claim is that they can detect a 20 percent relative pressure loss differential between tires, but others state they can only detect a 30 percent loss, e.g., a tire properly inflated to 30 pounds per square inch (psi) would have to deflate to 21 psi before the system would detect it.)  (9) some systems cannot detect a pressure loss at vehicle speeds of 70 mph or higher.


    Hybrid measurement systems

    The agency believes that an indirect measurement system supplemented with direct tire pressure measurement in two wheels and a radio frequency receiver, a "hybrid" system, could meet the proposal. This system was first discussed by TRW in its docket comment [6]. To date, no such systems have been produced.


    ALTERNATIVES

    In contrast to the June 5, 2002, final rule the agency is not proposing alternative levels of stringency. The proposal is that the driver must be given a warning when tire pressure is 25 percent or more below the placard pressure for one to four tires, or when tire pressure is at or below the defined minimum activation pressure (MAP).

    The MAP presented in Table II-1 shows the level at or below which the warning must be activated. The floor is different depending upon the tire type. All tires are required to have a single maximum inflation pressure labeled on the sidewall and that pressure must be one of the values indicated in the table. If a vehicle has p-metric tires marked 240, 300, or 350 kPa, it is a standard load tire that will be tested at 25 percent below placard, or 140 kPa, whichever is higher. If a vehicle has a p-metric tire marked 280 or 340 kPa, it is an extra load tire that will be tested at 25 percent below placard, or 160 kPa, whichever is higher. (Extra load tires are marked "XL" or "extra load" on the sidewall). LT-tires on light trucks have higher maximum inflation pressures and, therefore, have been assigned a higher floor below, which the warning has to be activated. The values in Table II-1 are the only values that can be used for maximum inflation pressure.


Table II-1
TPMS Lamp Minimum Activation Pressure
Tire type Maximum or
Rated Inflation
Pressure (kPa)*
Maximum or
Rated Inflation
Pressure (psi)
Activation
Floor (kPa)
Activation
Floor (psi)
P-metric  - Standard Load 240, 300, or 350 35, 44, or 51 140 20
P-metric – Extra Load 280 or 340 41 or 49 160 23
Load Range C (LT) 350 51 200 29
Load Range D (LT) 450 65 260 38
Load Range E (LT) 550 80 320 46

    * The standard is based on kPa, the psi values have been rounded to the nearest whole number.

    Currently, the lowest P-metric tire recommended placard pressure is 26 psi. At 26 psi recommended placard pressure, the 20-psi floor would come into play.   

    The rationales for the minimum activation pressure are:

    A 20 psi floor for p-metric tires is proposed because the agency believes that below that level, safety in terms of vehicle handling, stability performance, and tire failure is an issue. The agency ran a variety of p-metric tires in what it calls a "low pressure endurance test" at 20 psi with a 100 percent load at 75 mph for 90 minutes on a dynamometer. None of these tires failed. In a second set of test it calls a "low pressure high speed test" at 20 psi with a 67 percent load for 90 minutes, in 30 minutes steps at 140, 150, and 160 km/h (87, 93, and 99 mph), about 30 percent of the tires failed. Since tires could pass the "low pressure high speed test" at 20 psi, this leads the agency to believe that there will be a safety margin, in terms of tire failures, if a TPMS warning is provided at or above 20 psi, that will allow consumers to fill their tires back up before the tire fails, unless the vehicle is driven at very high speeds (above 140 km/h or 87 mph).

    The lowest inflation pressure used in the 2000 Tire & Rim Association Yearbook is 140 kPa (20 psi) for P-metric tires. In the 2001 Tire & Rim Association Yearbook, the 140-kPA pressures have been deleted, apparently because the Association believes they are too low for P-metric tires. The agency agrees that 140 kPA is too low and believes a floor is needed to assure that drivers are warned when tire pressure gets to or below that level. For the LT tires, we used the 2000 JATMA yearbook for the lower limits for Load Range C, D, and E tires. For most cases, the floor is about 58 percent of the maximum inflation pressure.

    For this Preliminary Regulatory Impact Analysis, the agency estimates the impacts of three TPMS systems that the vehicle manufacturers could use to meet the proposal (called "compliance options").

    Compliance Option 1 assumes that manufacturers will supply a direct system with either an interactive or continuous readout of individual tire pressures.

    Compliance Option 2 assumes that manufacturers will supply a direct system with just a warning lamp.

    Compliance Option 3 assumes that manufacturers with an ABS system would use a hybrid measurement system (indirect system with two direct tire pressure measurements) and vehicles without ABS would use a direct measurement system. We assume a warning lamp will be provided for drivers.

    New Issues and Analytical Assumptions
    For Compliance Option 3, we assume a hybrid system would be provided for vehicles that have ABS-systems currently (about two-thirds of the fleet). For vehicles that do not have an ABS-type system, we assume that a direct measurement system would be supplied. A direct measurement system costs less than adding ABS to the vehicle. A manufacturer could add ABS to the vehicle, but that is a marketing decision not brought on by the TPMS requirements.

    Maintenance Costs
    Since the court ruling, the agency has learned of advancements in direct system TPMS technology that have a large impact on the maintenance cost estimates the agency made in the March 2002, Final Economic Assessment. [7] A battery-less TPMS system [8] will soon be on the market.  This system will reduce the need for battery maintenance, since there will be no battery to replace, resulting in no quantified maintenance costs. For this analysis, the agency is providing a range from no maintenance costs for a battery-less direct TPMS system, to the estimates the agency previously used in its analysis for maintenance costs for a TPMS with a battery. One of the unknowns in the previous analysis is whether consumers would pay to maintain their TPMS systems, and keep achieving benefits in the later years of the vehicle. With a battery-less TPMS, this is no longer a concern. Comments are requested on whether a battery-less TPMS system will be the predominate design of the future.

    Malfunction/Warning
    In the March 2002, Final Economic Assessment, the agency assumed that all replacement tires would work with all of the TPMS systems and that the systems are maintained and reliable. This does not appear to be the case. As a result, the agency has decided to change its approach regarding replacement tires, and the NPRM proposes an additional requirement that was not in the June 2002 TPMS final rule.

    In the final rule, manufacturers were required to certify that their TPMS would work with any replacement tire that was of a tire size recommended for the vehicle. A number of vehicle manufacturers petitioned for reconsideration of this requirement arguing that they have no control over the replacement tire market and that a direct measurement system would not work with some tires. There appear to be three primary factors that might cause some replacement tires not to work with particular types of TPMS (i.e., carbon content of the tire, steel in the sidewall of the tire, and run-flat tires). First, the carbon content of the tire could cause sensor signal attenuation, rendering the TPMS inoperable. The carbon content is not labeled on the tire or available for consumers to determine before mounting the tires on the vehicle. Second, steel belts in the sidewall can also cause various levels of sensor signal attenuation. Steel belts in the sidewall are labeled on the tire but the labels do not provide information that would distinguish TPMS operability with those tires. Third, run-flat tires work with some TPMS but not others. Based on these findings, labeling does not appear to be a workable solution.

    As a result of the above considerations, the agency is proposing that each TPMS have a malfunction/warning system to indicate when the TPMS is not functioning properly, either because there has been a loss of power in the system, one or more of the radio frequency signals from an individual wheel are not being received by the control module of the system (signal attenuation), or for some other reason. The agency is proposing a malfunction/warning feature to alert consumers when the TPMS is not functioning properly, to help preserve the benefits.

    The agency is requesting comments on whether the malfunction/warning system should be a separate warning lamp or just provide a different warning signal using the same lamp. Currently, the low tire pressure warning lamp is required to come on when the system detects low tire pressure and must stay illuminated until the problem is solved. If the system detects a malfunction, the same warning lamp could, for example, blink or flash for 30 seconds or one minute (the agency is proposing one minute) each time the vehicle is started and then stay illuminated. This pattern would be repeated upon vehicle start-up until the problem is solved. The flashing lamp would give an indication that there was a problem with the TPMS and not a low tire pressure problem. The agency would like to have different indications for the two different problems (not necessarily the example given above), yet at the same time it would like to have a consistent message for consumers. Thus, we are asking for comments on how to best provide this information to consumers in a cost-effective manner.

    Similarly, if the alternative approved symbol is used, (i.e., the plan view of the vehicle showing all four tires), the symbol must stay illuminated until the problem is solved. If the system detects a malfunction, the symbol could blink or flash for one minute each time the vehicle is started and then stay illuminated until the problem is solved.

    It is not easy to determine the overall effects of this proposal for a malfunction/warning, since it is not known how large of a problem there is in compatibility between replacement tires and TPMS. A letter from the Rubber Manufacturers Association (RMA)[9] indicated in 2002 that light vehicle tires having either steel body ply cords (steel casing tires) or run-flat capability accounted for less than 0.5 percent of tires distributed in the United States. This estimate accounts for two of the three problem areas discussed earlier, although probably some of these tires will work with some TPMS systems, but it does not account for carbon content of the tire. In addition, the agency does not know the extent of other system malfunctions, like a broken sensor or antenna, for which the malfunction/warning lamp would provide benefits. We assume that they will be a small percent and will be subsumed in the overall 1 percent estimate. At this time, the agency’s best estimate is that if there were no malfunction warning, around 1 percent of the time the TPMS would not be working at some later stage in a vehicle’s life. This would occur either because the replacement tire designs would not work with a TPMS, or because there is some other malfunction with the system brought on by maintenance problems or mechanical/electronic failures. At the high end, the agency believes that less than 10 percent of tire designs would not work with a TPMS or will have other malfunction problems, but until the TPMS are designed and available for testing and the systems are on the road for years, there is no way of getting a better estimate, and there is no way of knowing how the replacement tire market could change in the future.

    If the agency requires a separate malfunction/warning lamp, then consumers who have replacement tires installed on their vehicle and get the malfunction warning could go back to the tire dealer and purchase a different set of tires. If the warning lamp stays lit until the system is fixed, the agency believes that most consumers will want to have their tires changed to extinguish the lamp, until they find out what it might cost them. The question is "Who pays the bill for mounting and balancing, and in some cases, the possibility that the second set of tires will cost more than the first set chosen." This could cost $50 or more. We assume this cost would fall upon the consumer, and not the tire dealer. If it is to be the consumer, we believe that many will ignore the lamp or have it turned off before they will pay another $50. We expect few consumers would go to the trouble of changing tires, just to have their malfunction lamp go off.

    For this analysis, we assume that the malfunction lamp will stay on and it lets consumers know that they have to check their tires themselves and cannot rely on continues TPMS operation. The big question then is "What percent of consumers will remember to check their tire pressure, given that they have a malfunction yellow lamp continuously lit on their instrument panel?" These are people that currently don’t check their tire pressure, or they wouldn’t be part of the benefits of the rule. The agency has no way of knowing this answer.

    The impact that a malfunction/warning lamp would have on benefits depends on what consumers do when they see such a lamp. The benefits of this proposal, safety benefits as well as tread life and fuel economy savings, are directly related to mileage. The average tread life was estimated to be 45,000 miles. The average weighted vehicle miles traveled was 126,678 miles for passenger cars and 153,319 miles for light trucks. That means that potentially 64 percent of the passenger car (1 – 45,000/126,678) and 71 percent of the light truck mileage will be driven on replacement tires. If 1 percent of the replacement tires are not compatible with TPMS designs, then an average of 0.675 percent of the benefits for both passenger cars and light trucks could potentially not be obtained if consumers were not provided with a malfunction lamp or if they ignored the malfunction lamp. Given, that the agency is proposing to require a malfunction lamp to rectify this potential loss of benefits to the extent possible, the main body of the analysis will not include a reduction in benefits for this factor. We believe the potential loss in benefits would be very small, less than 0.675 percent of the estimated benefits.

    Spare Tire Issues
    The above malfunction discussions do not consider issues dealing with spare tires and how spare tires work with TPMS. In the NPRM, spare tires are not required to be monitored by the TPMS. If a direct TPMS system is installed on a vehicle and a tire sensor is not included on the wheel of the spare tire, then when a driver gets a flat tire, changes it, and puts the spare tire on the vehicle, the malfunction lamp will illuminate. A TPMS malfunction lamp being continuously on is understandable when a temporary spare tire is in use. This could be considered a benefit, in that the driver would be reminded that the damaged full size tire should be repaired/replaced and the temporary spare tire should be stowed for future emergency use.

    However, if the TPMS does not work with a full-size spare, a malfunction lamp being continuously lit may become quite annoying. A malfunction/warning requirement might be a cost disincentive to supply a full-size spare because dealers may get a number of questions and complaints unless the spare tire is set up to work with the TPMS system.  

    Manufacturers may decide to put a TPMS tire pressure sensor in the spare tire, to avoid consumer complaints in the future, at a cost of $7.50 per wheel. Since 15 percent of the vehicles come equipped with a full-size spare tire, the total cost if all full-size spare tires had a tire pressure sensor would be $19 million (17 million vehicles *.15 * $7.50). Depending upon how much manufacturers value not having consumers complain about their TPMS and spare tires, they might decide $7.50 is a worthwhile investment. The analysis does not assume that manufacturers will put a tire pressure sensor in the wheel of full-sized spare tires. The analysis does not estimate a cost for the inconvenience of a having a continuous malfunction/warning lamp on, caused by a spare tire without a tire pressure sensor.

    Additional Alternatives/Compliance Options
    NHTSA has not examined an indirect system in this analysis. However, it is possible that an indirect system could be developed that provides up to 4-tire capability. To date, the agency is not aware of any indirect system that is available that has the capability of activating reliably at the proposed trigger level of 25 percent below placard, nor do we know what the costs would be of such a system. NHTSA requests comments as to the capabilities and limitations of indirect systems in meeting the proposed requirements of this rule. Specifically, we are interested in knowing whether such systems would be able to detect all combinations of pressure differences on individual tires, the technical aspects of how such systems would operate, the cost of such systems, and what restrictions they would face in detecting specific levels of pressure reduction.

    Although NHTSA is proposing a 25 percent below placard threshold, technically, other threshold levels could also be established. Selecting a notification threshold level is a matter of balancing the safety benefits achieved by alerting consumers to low tire pressure against over-alerting them to the point of being a nuisance, such that they ignore the warning and defeat the safety benefits of this proposal. We cannot predict a specific threshold level where benefits are maximized by a combination of minimum reduction in placard pressure and maximum response by drivers. However, degradation in vehicle braking and handling performance doesn’t become a significant safety issue at small pressure losses. Moreover, NHTSA is confident that existing technology can meet the 25 percent threshold. Setting a lower threshold might result in the opportunity for more savings if driver’s response levels were maintained; however, we are concerned that setting a lower threshold could result in a higher rate of non-response by drivers who regard the more frequent notifications as a nuisance. Current direct TPMS systems have a margin of error of 1-2 psi. This means, for example, that for a 30-psi tire, manufacturers would have to set the system to provide a warning when tires are 4 psi below placard if we were to require a 20 percent threshold. In some circumstances, overnight temperature declines can temporarily reduce tire pressure by 2-3 psi, but normal pressure would be restored as the tires heat up during use. This is not the type of pressure decline that TPMS is intended to address, and repeated nuisance alarms could result in reduced driver response to actual low tire pressure events. We have not examined lower threshold levels in this analysis because we believe that the net impact of these offsetting factors (quicker notification, but lower frequency of driver response) is unknown and unlikely to produce a significant difference in safety benefits. We note that a 20 percent 4-tire option was examined in the March 2002 analysis, and that the total benefit for the 20 percent threshold was about 15 percent higher than from the 25 percent threshold. However, that calculation assumed the same level of driver response for both thresholds. It is also possible that lower thresholds may limit technology and discourage innovation.

    Overall, we feel that the 25 percent threshold adequately captures the circumstances at which low tire pressure becomes a safety issue. We also believe that this level would be acceptable to most drivers and would not be considered a nuisance to the point that it would be ignored by large numbers of drivers. We also believe there is no reason to examine higher thresholds (e.g.,  a 30 percent threshold), which would provide fewer benefits for similar costs.



    [2]  Published in the Federal Register on 6/05/02 (67 FR 38704), Docket No. 8572-219.

    [3]  Public Citizen, Inc. v. Mineta, 340 F.3d 39 (2d Cir. 2003).

    [4] However, it may be possible to meet the 25 percent one to four tire proposal with an indirect system, that takes more measurements than the current indirect systems, as technology evolves.

    [5] March 2002 "Final Economic Assessment, Tire Pressure Monitoring System, FMVSS No. 138", Docket No. 8572-216.

    [6] Docket No. 8572-110.

    [7] March 2002 "Final Economic Assessment, Tire Pressure Monitoring System, FMVSS No. 138", Docket No. 8572-216. 

    [8] See IQ Mobil docket submission No. 8572-318.

    [9] Letter from Steven Butcher, Vice President, Rubber Manufacturers Association, to NHTSA (October 31, 2003) (Docket No. NHTSA 2000-8572-282). 


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