Ding! A symbol that looks like a baseball bat dropped into a punchbowl appears on your dashboard. You have a tire that’s low on air, and your car’s alerted you to it far before the heat and friction of underinflation cause odd tire wear or a panel-bending blowout. Well, either that or one of your tire pressure monitoring sensors has died. But what if there was a better way of keeping tabs on tire pressure? One that perhaps trades some precision for being completely maintenance-free? Well, such a thing used to exist, then the federal government killed it, but now it’s coming back. May I introduce you to indirect tire pressure monitoring?

The tire pressure monitoring systems in most American cars feature four sensors and a receiver, or sometimes four sensors and a combination receiver-transmitter unit. See, those expensive little sensors are jam-packed full of tech, from the battery to the pressure sensor to the analog-digital converter to a radio frequency transmitter sometimes used in conjunction with a receiver inside each sensor to share data with the tire pressure monitoring system receiver or receiver/transmitter inside the vehicle. This system of wireless data transmission explains why you could theoretically build a cheap pressure chamber out of pipe and a schraeder valve, dump four sensors in there, pump it full of air, throw it in the trunk, and the TPMS system will be none the wiser. It also explains why these things cost a mint, adding three figures to the cost of, say, a winter tire and wheel package.

However, indirect tire pressure monitoring doesn’t add any hardware to a car, so they should be cheaper even factoring in the software development (and tire changes should be easier). In fact, an indirect system just takes signals the car’s already generating and uses them to discern if a tire’s low on air. An underinflated tire will have a smaller overall diameter than properly-inflated tire, which should get picked up on the wheel speed sensors used for ABS as a difference in wheel speed. If the difference is great enough, the vehicle’s electronics will notice and illuminate a warning light on the dashboard. In principle, “indirect” tire pressure monitoring is perfect for anyone whose last name isn’t Verstappen or Vettel. There are no expensive wheel-mounted sensors that eventually crap out or may get damaged in the tire mounting process, and winter tire swaps are extremely easy. However, there’s a reason why indirect tire pressure monitoring briefly disappeared from the American market.

In 2005, the federal government mandated a phase-in of tire pressure monitoring systems in every car, with a firm deadline of September 2007. However, talks about mandating TPMS started years earlier, due to a headline-making scandal involving Ford Explorers and Firestone tires. Underinflation, blowouts, and a whole lot of mud-slinging over a recall prompted Congress to pass the TREAD Act, a piece of legislation primarily focused on recalls and reporting. However, section 13 of the TREAD Act specified that:

Not later than 1 year after the date of the enactment of this Act, the Secretary of Transportation shall complete a rulemaking for a regulation to require a warning system in new motor vehicles to indicate to the operator when a tire is significantly under inflated. Such requirement shall become effective not later than 2 years after the date of the completion of such rulemaking.

Well, so much for the deadline. Eventually, this tiny little section of the TREAD act led to Federal Motor Vehicle Safety Standards section 138, a tire pressure monitoring system mandate with very specific criteria. A critical part of the test certifying tire pressure monitoring systems for use on American vehicles is to:

Stop the vehicle and deflate any combination of one to four tires until the deflated tire(s) is (are) at 7 kPa (1 psi) below the inflation pressure at which the tire pressure monitoring system is required to illuminate the low tire pressure warning telltale.

Sounds like simple criteria, right? Unfortunately, most older indirect TPMS systems can fail this test. From NHTSA:

There are two types of TPMSs currently available, direct TPMSs and indirect TPMSs. Direct TPMSs have a tire pressure sensor in each tire. The sensors transmit pressure information to a receiver. Indirect TPMSs do not have tire pressure sensors. Current indirect TPMSs rely on the wheel speed sensors in an anti-lock braking system (ABS) to detect and compare differences in the rotational speed of a vehicleís wheels. Those differences correlate to differences in tire pressure because decreases in tire pressure cause decreases in tire diameter that, in turn, cause increases in wheel speed.

To meet the four-tire, 20 percent alternative, vehicle manufacturers likely would have had to use direct TPMSs because even improved indirect systems would not likely be able to detect loss of pressure until pressure has fallen 25 percent and could not detect all combinations of significantly under-inflated tires. To meet the three-tire, 25 percent alternative, vehicle manufacturers would have been able to install either direct TPMSs or improved indirect TPMSs, but not current indirect TPMSs.

As I mentioned before, early indirect tire pressure monitoring worked simply by referencing the speed of all four tires against each other using a vehicle’s wheel speed sensors. The theory is that an underinflated tire has a slightly different diameter than a properly inflated tire, and will therefore spin at a different speed. While this eliminates pesky TPMS sensors, it does come with a blind spot — if the pressure in all four tires is reduced by the same amount, the TPMS light won’t come on. After all, if all four tires are rotating at the same speed, a computer only referencing relative wheel speed won’t notice anything’s wrong. Early indirect tire pressure monitoring also has a limitation when it comes to certain staggered tire setups that run different overall tire diameters on each axle. For those vehicles, similar deflation across any one axle may cause a false negative.

However, since the advent of FMVSS 138, indirect tire pressure monitoring systems have evolved substantially, with Brake & Front End reporting that “New systems are taking advantage of better wheel speed sensors and modules to make indirect systems work.” A specific example is how Infineon Technologies has a patent on indirect tire pressure monitoring using “an electronic control unit (ECU) coupled to the ABS and configured to process the sensed signals using a multidimensional resonance frequency analysis (MRFA) that includes a spectral analysis identifying at least two tire vibration modes in the wheel speed signal and isolates at least one characteristic affecting the at least two tire vibration modes.” Translation? By using spectral analysis, this system can detect if all four tires are losing pressure simultaneously.

Modern Tire Dealer, a website that’s apparently the “premier source of news, research and market trend analysis,” breaks all this down further, discussing Audi’s new indirect system from supplier Nira Dynamics AB called “TPI.” After noting that it’s easy enough for an indirect system to use wheel sensors to figure out if one, two, or three wheels are deflated (since the tire radius changes, and thus the deflated wheels will have to rotate faster to “keep up”), the issue of four simultaneously-low wheels comes up:

TPI also features other components analyzing high-frequency components of the wheel speed signals stemming from different vibration modes of the wheel. These approaches are applied to each wheel separately, and by combining this with the previously mentioned radius analysis, TPI can detect under-inflation in one, two, three and four tires simultaneously.

The article goes on to say that there’s an “initial calibration or learning phase” to figure out what the normal “characteristic parameter values” should be with fully inflated tires. Those are later used to compare what the vehicle is reading from the sensors as a user drives the vehicle. “The driver should reset the system and re-initialize calibration whenever he or she has made a change to the tires, the tire pressures or, in general, to the wheels,” the article continues before saying:

TPI is designed to give a warning for under-inflations of 25% or more compared to the nominal pressure level, which the system learns during the calibration phase (25% is the required detection level under FMVSS 138).

The best statement regarding the reliability of the system at this point is probably that TPI (has been) in production for the Audi TT since 2006 and for the Audi A5/A4… and has received positive reactions from customers and quality assurance people alike. Further, since TPI is a software-based system using existing and well-proven components of the ABS/ESC system, in particular the wheel speed sensors, it’s optimally simple and robust. The key point here is that software doesn’t break!

These developments allowed these indirect systems to slowly start trickling back into the American market in the 2010s. The first-generation Mazda CX-5 (see above) used an indirect tire pressure monitoring system, as do many Volkswagen Group cars and most newer Hondas. For the vast majority of drivers, it’s simply a better, more cost-effective, maintenance-free solution compared to direct tire pressure monitoring.

So what if you’re a tire pressure gauge-carrying scofflaw with a car from the late aughts and don’t need no stinkin’ TPMS sensors? Well, depending on the make and model of your car, you may be able to “code in” indirect TPMS. For instance, most Bangle-era BMWs will let you code in Flat Tire Monitoring, an indirect tire pressure monitoring system that uses relative wheel speed derived from wheel speed sensor data instead of an expensive tire pressure monitoring sensor in each wheel. Of course, we don’t advise breaking any laws, but we do think it’s funny that tire pressure checks used to be part of normal driving routine, and now many people simply don’t carry a cheap tire pressure gauge in their car.

(Photo credits: Thomas Hundal, Amazon, Mazda)

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