saw a new car that uses tire pressure monitoring using the ABS system by measuring wheel rotation. if a tire is low, the diameter is less and the computer will sense this. so, no wheel pressure sensors are used. a standard tire has about 10/32 of tread. if 1 tire was at wear bars, or less, it would be about 7% smaller overall, so i imagine the system would think the tire was going flat? yes i am thinking of 1 worn tire and 3 good tires.
It might. It depends on the tolerances built into the TPMS system. Of course, if you’re driving around on one worn tire and 3 new ones, then the tire shop ripped you off and it would be cool if TPMS alerted you to it 
That said, I don’t like rotational TPMS. Too much room for inaccuracy. Since tires will lose pressure at pretty close to the same rate as temperatures drop, you could be running 15 pounds in all 4 tires and TPMS would never know it. I much prefer the individual wheel sensors. The good ones will display your tire pressures for you on a screen in the car. The really good ones don’t require re-calibration when you rotate the tires because they automatically sense their position on the vehicle.
I think your math is off. My car has 235/45-18 tires that start at 10/32 tread. The diameter when new is 26.33 inches. The diameter when at the bars (2/32 tread) is 25.83 inches. That’s a difference of only 1.9%.
Mine works that way. These systems can and should be “reinitialized” (NOT simply “reset”) whenever new tires are installed, with the tires set to the desired pressures, to set new baselines. It then simply operates on the comparisons with the baseline settings for each wheel. It works great. Mine notified me when I had a slow leak due to a screw before the tire became too low.
"That said, I don’t like rotational TPMS. Too much room for inaccuracy.“
Gotta dusagree with you there. With no sensors in the wheel, this TPMS doesn’t add expense the way the pressure-tranducer does. Gotta love affordability! More importantly, many states do not require annual inspections…and many states that DO inspect, don’t fail for inoperable TPMS. That means that a battery-based TPMS system is most likely just a big “brick” on most cars > 7 y.o.
So, “cheap, reliable TPMS that actually works” > " expensive, INOP TPMS” in my book…even though I agree with you that the pressure-based TPMS is a technically better setup.
Gotta dusagree with you there. With no sensors in the wheel, this TPMS doesn't add expense the way the pressure-tranducer does. Gotta love affordability!
What does affordability have to do with accuracy? I know the pressure sensors are more expensive, but they also tell you what the actual pressure is rather than just telling you “Hey, one of your tires is a different pressure than the others, but I’m not going to tell you which one, and if all of them are equally low I’m not going to tell you at all.”
If you replace the batteries when you get new tires, they almost never run out of power unless something weird happens. If we’re going to claim that a system is bad just because negligent owners fail to maintain it, then we could also say that engine lubrication is useless because some owners are dumb and never change the oil. 
Affordability has nothing to do with accuracy. It has everything to do with maximizing “bang for buck.”
If the most desirable option is always the most technically advanced, then it’s a no-brainer: design for overkill, costs be damned, and everybody would drive around in Mercedes. In the real world, performance and function has to weighed against cost. A Civic certainly isn’t a “better” car than a Mercedes, but it usually is a “better” choice for a daily driver.
C’mon…this concept is familiar to everybody who’s ever had to budget! Agree or disagree with my priorities…but don’t play coy and pretend you don’t understand the concept. That makes you sound like Mme Antionette*, who couldn’t figure out why folks simply didn’t eat cake, if they had no bread handy!
And by “become a brick after 7 years,” I mean I own a car-based TPMS (on a 6 y.o. car), have no intention of spending one red cent on it…and thus understand that its days of providing useful information are limited. That’s okay with me…“neglecting” this feature just means that the car will function akin to every other car I’ve ever owned, and compromises no other systems on the car…so if not mandated, why not just go with the “time-proven” solution of a tire gauge?
- (Yes, I realize this is a mis-attributed quote!)
“If you replace the batteries when you get new tires…”
LOL, which car with in-tire sensors lets you do that? Those batteries will cost you $100s for the entire sensors. I use black tape on my failing Insight TPMS system dash lights (yes, there are two). They wouldn’t tell me which wheel anyway. I’m not going to pay more maintaining my TPMS than the tires cost!!! Honda has now switched to an ABS-based system.
Well, yes, I meant the sensors - When they die, you change them at the next tire change. I find TPMS to be useful when it reports the pressures. I find it to be useless when it does not, because as we’ve discussed, TPMS systems that are rotation-based have no clue that you’re running on grossly underinflated tires if all 4 of them are grossly underinflated. What’s the point? Every winter people with TPMS systems are running around on dished-in underinflated tires because they assume TPMS will, like, monitor the pressure in their tires. That’s stupid.
And speaking of dying, I have almost 100,000 miles on mine and they’re still doing just fine. It’s not like they run out of power every year.
I think it’s funny that on a forum where the regulars pretty routinely bash people for not spending the coin to properly maintain their vehicles think it’s stupid to spend the coin to properly maintain their TPMS and would rather have a grossly inferior system that creates a safety hazard by instilling a false sense of security.
I’ve personally been, in the past, berated for choosing not to spend $2,000 on a winter wheel/tire package, but choosing to use and maintain a proper TPMS system that actually works makes me some sort of cake-eating elitist. That’s bizarrely inconsistent.
Has anyone had to replace the battery in one? My 4runner is approaching 10 years now and not one failed yet.
If only…
The batteries are not replaceable ! They’re there ( some 2025s and 2032s ) and I’ve dug them out of the glue they’re burried in but they’re soldered in and covered in a rubbery goo.
When TPMS was mandated, yes mandated on all new cars from 2007 up, the early indictive ABS wheelspeed sensors were not considered accurate enough to meet the NHTSA’s standard. Active pressure sensors needed to be installed in the wheels to meet regulation.
Now ABS sensors may be used for TPMS. BUT and a big BUT, they only read relative measures of rotation from side to side and front to rear. They ALL could be low and no warning will be set unless an on-board GPS is used to absolute vehicle speed. Given that it is likely all new cars have a GPS chip somewhere in the system, this seems quite possible, now. You would still need learning algorithms to adjust for wear and new tires.
Well when the light goes on,I go a looking-The things arent that expensive from Rock Auto,a good honest shop wont take you to the cleaners to reset em-Kevin
If you replace the batteries when you get new tires, they almost never run out of power unless something weird happens.Yep. That's the downside, right there: 50% price increase on tires!
If we're going to claim that a system is bad just because negligent owners fail to maintain it, then we could also say that engine lubrication is useless because some owners are dumb and never change the oil.
Now THAT’S an interesting concept.
I, as a motorist, have an obligation to drive a car that is roadworthy and safe in operations. Part of that is not running on grossly over- or under-inflated tires. Up until now, I always assumed HOW I ensure that is up to me: do I use TPMS, tire gauge, etc?
But…this raises the concept that not only must I ascertain tire pressure, I must do it by a proscribed means, not to be negligent. I wonder if you can get a second on that? (Seems quite a bit too “micromanaging” for me, but maybe that’s just, like, my opinion, man.)
P.S. I never meant to say you were an elitist: I meant to say you were acting as if you had no working concept of scarcity. Marie Antionette is the only (mis)quoted public figure I know of who failed to grasp it. Perhaps there are others, though…
Personally I don’t find checking the tire pressure after a fill-up once in a while to be much of a burden. And it gives me an opportunity to visually inspect for tire problems not flagged by a TPMS, like embedded nails, cupping, abnormal wear patterns, sidewalls cracking, dodgy valve stems, missing valve stem caps, etc. But if checking the tires was a burden for some reason, I guess a car sporting the rotation speed version of TPMS is probably quite a bit better than a car having no TPMS at all. I wouldn’t worry about it, even if it wasn’t quite as accurate as the in-tire-sensor version.
Mustangman, wheelspeed-sensor based TPMS systems don’t compare the wheel speeds with one another; they compare each with a “baseline” setting for that specific wheel entered into the ECU (all at once) via an “initialization” process (not to be confused with “resetting” the system). Active pressure sensor were not needed to meet the regulations, the wheelspeed-based system did that. Pressure sensors were simply an alternate approach. I have Toyota’s design documents and “initialization procedures” for the system.
Pressure sensor are considered by many to be better because they monitor the spare tire too, but that’s not a D.O.T. requirement. Cars are not even required to have spare tires, and many new sports cars don’t. They provide a can of “fix-a-flat” instead. I don’t agree with the practice, but there’s no requirement to do otherwise.
Sounds like a good system to me(a lot better then none)-Kevin
A set of 4 aftermarket sensors for my car are $41 from Amazon. With at least a 5-year battery life, that’s under $10/year and the system displays tire pressures. A full set of OEM Mopar sensors are only $90, still very reasonable with a 5-year life. Most tire shops will of course mark them up a bit, but I don’t consider this very expensive at all for the peace of mind you get out of it.
This is a valuable system that gives you early warning of problems, makes it easy to top up your tires by just glancing at the dash for pressures, and you can hardly say that maintaining the system is expensive. Even if you pre-emptively replace the sensors every 2-3 years, that’s still dirt cheap.
@TSM, if the wheelspeeds are compared to a value in the ECU, what is that “baseline” value? There is no absolute reference speed unless GPS (or a fifth wheel) is involved. All speeds, including the speedometer sender are relative references based on the rotation of the wheels. There is no “real” speed available. Comparing the the speedometer only gives another relative speed that matches the ABS speeds off the drive wheels.
Comparisons to each wheel is how ABS works, too. One wheel slowing too fast (to lock), compared to the other 3, while braking, releases pressure to that wheel. A similar algorithm is used to recognize low pressure, without indicated braking.
I was working in engineering with ABS engineers from GM’s brake division, at the time the regulation was set in place. Active sensors inside the wheel were chosen, by GM, because the wheelspeed sensors were not sensitive enough to provide TPMS per the requirement the government imposed on carmakers. The added expense for pressure sensors in the wheels would not have been chosen at the time if GM could have used the ABS sensors for that task.
Interesting that the batteries are so expensive for some people. Last time I was at a Discount Tire in Washington, they guy at the desk mistakenly specified that I had a TPMS on the work order. I had to stop the tech from putting new valve stems in with built-in sensors, and had him just put new regular stems instead. So it seems like the sensor/battery combo for some cars at least costs very little if tire shops are happy to change them out automatically every time they do a tire job.