Another esteemed member of this forum (AstroKats) brought up the fact that the engineer who designed the dash's mpg readout system probably had a hand in your TPMS. My TPMS seems to do what it's supposed to in giving a ballpark reading. But yours isn't even in the ballpark.

 

After adding/removing air from the tires you have to drive a few miles to get a new reading. Mine seems to match either of the gauges I use but they are never very accurate either. Outside temps and driving change the readings as well. You should test it a few more times before deciding there is a problem.

It's like a lot of the new technology, it's useful but we complain when it doesn't work like we imagine it should. My wife would never check her tires like most drivers never do. Now she has a light that goes off in her +12 Outback and I have actual numbers on the dash to go along with a low tire warning. It makes me more aware of the condition of the tires which I wouldn't think of for months at a time with my old car. So now I have a cool feature that reminds me to pay attention to the tires but if it seems off I get annoyed.

 

I didn’t add or remove air since I got the car (3months). So I would assume the dash readings would be in sync by now.

 

So that was probably the reading when you got home the day before after driving the car. In the morning when the tires were cooler the pressure was lower so the two readings make perfect sense. Seeing as how hot it has been I'm not surprised the readings were on the high side. Charles's law explains it.

 

Doesn’t make any sense. I started up the car after manually checking the tire pressure and the dash gage was still high. Are you saying it was yesterday’s readings.

 

Yes, like I said earlier...you have to drive a few miles to update the reading.

 

Tire pressures vary a LOT depending on outside temperature and driving conditions. TPMS isn't designed to be spot-on accurate, it is designed to warn you of an unsafe condition primarily caused by low tire pressure. It also isn't giving you real-time readings, the sensors only transmit occasionally (I don't recall the frequency) while the car is moving. I find that the pressures reported by the TPMS on my car are generally fairly close to the actual pressure once things sync up. (I've checked them with a gauge I trust.) If they really seem way out, check the tires against the TPMS readings after you've driven a few miles.

 

Thanks chuck. After driving around some today the dash readings became close to actual. I thought they would be real time readings. So you can’t use these as a gage if adding air. My Acura had real time readings. If you were filling the dash gad3 would reflect immediately.

 

As others have posted, you can't use them as a tire gauge while pumping up the tires. After you've driven for a few minutes it should update, and will change with temperature (i.e. how far you have driven and warmed up the tires, etc.)

They are not super accurate. Ours read 1 or 2 PSI above the actual, according to my tire pressure gauge (verified by the tire shop this morning - more on THAT in my next post... :icon_rolleyes

 

I have a really weird issue, only 1 tire gets a warning saying it’s way under inflated - 23 lbs. With a gauge it was showing down to 30 pounds, so overinflated it to 36, according to gauge it was holding. TPMS light would still say low randomly. Put spare on and took tire to tire shop to check for leaks- none. Put back on car read 36 lbs. Next day low tire (but it’s not). Before I head to dealer, any way to reset sensor? Husband heard you can do that sometimes. Car is a 2018, would not think sensor would go bad that fast!?!

 

Unless you're over 36K miles I would imagine the dealer would cover it under warranty.

 

The wild variance in pressure in the display and it's difference from what the gauge says means something is wrong somewhere.Start by setting the pressure with a standard pressure gauge.Drive at highway speed for at least 10 miles to insure the display has an up to date reading.If the display shows pressures that don't seem right the take it to a tire shop.Have them check the pressure with their TPMS scanner.If the pressures read right then the problem is with the car.Either it needs to be calibrated or a part needs replaced.If the scanner reads those wild variances then the sensors are not accurate and need replaced.I have a TPMS scanner and my readings are always within =/- 0.X psi of what a standard pressure gauge reads.

 

Thanks, We have done all that (although I’ll have to check on whether the tire shop used a scanner), then it sounds like next stop is the dealer. Thanks for the input!

 

TPMS only works when you are driving. They "wake up" and start sending signals to the car. Some cars are better than others at accuracy but they will be close. Also you can have a hi or a low system in cars. Low will only measure when psi is below a threshold (usually 25% below door placard) and the light will come on. High line system will actually give you the psi. Car has to be moving for the signal to be sent.

 

I think the main thing is that the TPMS can tell you if a tire is wildly out of line and might cause a problem with your vehicle stabilty and braking due to unequal forces. It never occurred to me to think the things were dead on accurate. I would agree with earlier comment that the folks that programed your driving mileage computer also programmed your TPMS...but that would mean my tires would always show much higher tire pressures than they actually had.

 

When I picked up my car from the dealer, the TMPS readings were all showing 40. I let the car sit overnight, and checked the pressures with three different gauges - two stick type, and a known accurate accutire digital gauge. I adjusted the fronts to 33, and the rears at 32 per the door plate. The TPMS display shows 30 and 29 respectively, so it's pretty much on a -3 and it is consistent. Not sure if it's a problem I should be concerned about or not.

My dealer puts nitrogen in the tires (high altitude, and large temp swings depending on season), but pressure is pressure. I wouldn't think that would affect the sensors at all, but I am sure someone knows more than I do on that subject.