Just how accurate is the tire pressure monitoring system?

I've been wondering what the tolerances are, too, considering the pressure drops about three to four psi each time I pull out of my garage.

When I test drove a Prius the TPMS light came on but the tire looked within psi specs.

I saw a guy at Kroger tonight with abismal pressure and wished he had TPMS..or at least some eyeballs and a brain. Both starboard tires looked to be about 15psi.

You should always check your own psi and carry a reliable gauge with you - you can never be too sure with those built-in gauges at the coin air pumps.

 

TPMS is designed to warn the driver about flat tires. It is not a substitute for checking tire pressure on a routine basis. I don't know the exact specification for the Gen III, but typically TPMS will trigger at somewhere between 10% to 20% drop in pressure.

Tom

 

I reset my tpms system at 46 f and 44 r, then I dropped the pressure to 42 f / 40 r. A 15% drop trigger would be around 39 f/ 37r. That ( in my opinion) would be a great warning to re-inflate to the target 42/40. That is if it is consistent.(and accurate) Dave

 

The top line TPMS in other autos will show you the actual pressure in each tire real time and alarm if pressure is out of range. However any TPMS is of great value. More tires are blown out from slow leak puncture, get hot from low pressure and then come apart. No TPMS will help you if you run over a shovel and cut the tire down all at once. TMPS systems have been around a long time (vettes for over 10 years). Again the auto companies did not think we needed it until the gov mandated. They talk safety but dont walk the walk sometimes. That sounds like the company I work for.

 

I just used the TPMS reset button a few days ago and had the TPMS service data read with the Toyota Techstream tool. So the tire pressures may have changed a bit, but not much.

My goal pressure was 42 front, 40 rear. I set them as close as I could with an analog dial pressure gauge. The Techstream readout turned out to be 42.1/41.7 front, 39.5/41.7 rear. According to the TPMS system, the "Threshold of Low-pressure" values (level at which the TPMS light will come on) are 33.7/33.0 front, 31.5/33.4 rear. So it appears that the reset function sets the warning level to about 80% of the current level. And either I messed up with one of the rear tires, or the sensor in it is off by 2 pounds.

So to get a warning pressure of 33.7, I had to set the real pressure to 42.1 and press the reset button. I'd actually like the light to warn me a bit sooner than that. Since I have to recheck those rear tires anyway, maybe I'll aim for a warning pressure of 39 front, 37 rear. That will require 52 front, 50 rear. Then I'll drive around for maybe 5 minutes at that pressure to make sure the TPMS ECU gets the new values. Then I'll press the TPMS reset button to have the ECU recalculate the warning levels. Drive around for another 5 minutes. Then reduce the pressures back down to the 42/40 that I actually prefer.

Think my wife will buy this project as reason to not cut the lawn?

 

"The word" from Toyota is the TPMS will warn you if any tire pressure drops more than 20% from the value they had when you set the system by holding the button in until it beeps at you. In my experience it works as advertised. It's a bit of a pain, keeping the tyres at the correct pressure. And having to respond to a tyre with a leak in it when it's -25C. But it's better than destroying a tyre and wheel and perhaps a car.

 

Good thread! I just came into the technical discussion to search for the answer to this question, and there it was without a search required.

I bought my car on 1/29/11, and decided I should check my pressures. Every tire was at about 28 psi! (And I didn't get any TPMS light). Apparently the pre-delivery inspection didn't include a tire pressure check. I set them all at 38 psi as a starting point and reset the TPMS. I am wondering how much my first tank mileage suffered as a result of underinflated tires. Isn't there a rule of thumb for every 1 psi you can lose 2-3% mileage?

Thanks
Jeff

 

Hey Jeff, that's awesome about the new car! While I can't answer your question, you should note that because of a little extra weight in the front, it's generally suggested that Prii have 2 psi more in the front tires than in the rear.

 

Thanks for the feedback! I will make an adjustment and reset the system. Now the question - better to go 40 front/38 rear, or 38 front/36 rear? I get nervous about too much overinflation and the risk of uneven tire wear...

The recommended pressures (from the driver door jamb sticker) are 35 front 33 rear I think...

 

I just got mine early December, so I won't even pretend to speak from some position of experience. After reading around here it seems many (most?) do it regularly with no horrible adverse effects, so I am at 41f/39r myself since the beginning of January or so, and did notice a small pop in mileage.

My impression is that the ride might've gotten slightly more bouncy with the higher inflation, but not anywhere near odd enough that someone riding with you would think there's anything wrong.

If you have the original tires, the max sidewall is 44 for cold weather, so I wouldn't be nervous about trying 40/38. If 40 is a psychological barrier, go for 39/37

 

Thanks - just reset at 40/38

 

If you read the tyre sidewall carefully, you will notice it says "max. -load- at xx psi xxxx lbs". While -I- wouldn't suggest exceeding this pressure, it is actually a tire -load rating-, not a max pressure rating. If you inflated the tyre to the max load rating pressure at "room temperature" (72F or 20C) where it's supposed to be for this measurement, then drove at highway speeds on a hot day, you would find the tyre pressure to be quite a bit above this rated number. It's designed to be that way. So the tyre can take considerably higher pressure than this number on the sidewall. Please don't test this. We've already had the report of the lady with 90 PSI in her tyres, which caused one to explode and blow off the front fender!
At any rate, the number on the sidewall is safe to use, providing you inflate at room temperature.
The number on the door plaque is what the "experts" want you to use. It's chosen for comfort and safety (tyre not "underinflated").

Higher pressures, up to reasonable amounts, are ok, they will just give you a firmer ride. If you get into the uppermost areas of pressure (still under or at the max load pressure) you -may- find the tyre will wear faster in the centre of the tread. Modern tyres do resist this effect very well, but it's something to watch for.
Higher pressures will improve mileage, improve snow clearing, and -may- improve handling. YMMV.

 

Sounds like the one I used to work for.

 

The EU specs for TPMS are pretty clear - and I'm sure Toyota goes by it...

Some snippets from EU Reg 64:
5.3.2. Tyre pressure detection for incident-related pressure loss (puncture test)
5.3.2.1. The TPMS shall be tested according to the test procedure set out in paragraph 2.6.1 of Annex 5. When tested to this procedure, the TPMS shall illuminate the warning signal described in paragraph 5.3.5 not more than 10 minutes after the in service operating pressure in one of the vehicle’s tyres has been reduced by 20 per cent or it is at a minimum pressure of 150 kPa, whatever is higher.

5.3.3. Detection for a tyre pressure level significantly below the recommended pressure for optimum performance including fuel consumption and safety (diffusion test)
5.3.3.1. The TPMS shall be tested according to the test procedure set out in paragraph 2.6.2 of Annex 5. When tested to this procedure, the TPMS shall illuminate the warning signal described in paragraph 5.3.5 within not more than 60 minutes of cumulative driving time after the in- service operating pressure in any of the vehicle’s tyres, up to a total of four tyres, has been reduced by 20 per cent.

5.3.4. Malfunction detection test
5.3.4.1. The TPMS shall be tested according to the test procedure set out in paragraph 3 of Annex 5. When tested to this procedure, the TPMS shall illuminate the warning signal described in paragraph 5.3.5 not more than 10 minutes after the occurrence of a malfunction that affects the generation or transmission of control or response signals in the vehicle’s tyre pressure monitoring system. If the system is blocked by external influence (e.g. radio-frequency noise), the malfunction detection time may be extended.

I'd say it's 20%...