Minok

Now that I've driven the car a few months, I've noticed the drivers front tire is 2psi below the passengers front.. the backs are still the same. I'd not made any tire pressure adjustments since it was delivered new from the dealer.

Sitting in the garaged car, I pulled up the TMPS system to get to the screen that the manual indicates will tell me how much I need to adjust each wheel by based on the programmed wheel type/size. Well thats a surprise. The readout says above pressure on all 4 wheels... 7+ on the backs and 2+ on the fronts on average.

So my question is... did the dealer deliver the car with too much air in the tires (on winter 18" tires)? (too much air in cold weather situation)

Or is this whole temperature compensation system in the TPMS overreacting?

Tires were cold (sat all night in the garage with an ambient temp around 48 degrees F) Its reading the same differntial now in the sunny parking lot at work after sitting for 7 hrs (ambient around 58-60 F).

+2 === +4

+7 === +7



So do I use my external gauge on the valve and fill to the ratings on the door sticker, or do I do what the TPMS recommends and let some air out of the tires respectively.

I''m not looking for track performance but daily driver, let the tires wear well performance.

Bob Rouleau

Probably the dealer overfilled the tires or maybe it was 20 below that day. TPMS is accurate to around 2 lbs or so. Good enough to warn you of a flat. I have never seen an indication of too much air, it may not report it.

Best is to set pressures with an accurate gauge to the pressures suggested for your tires. Note, pressures for 18 inch tires will be in your manual. If your car came on 19 inch wheels the pressures on the door sticker may not apply to 18 inch versions. Likewise if the dealer fitted narrower winter tires, the info will be in your owners manual.

Best,

RonCT

The new Gen 2 TPMS are supposedly much quicker and accurate. I'll find out when my 2009 C2S arrives. But I've never trusted TPMS other than to warn me of unexpected change (pressure loss while driving). I always use my Longacre and have found old Gen 1 TPMS to be as much as 5 PSI off. Easy way to test this - adjust the pressures to your gauge and see what TPMS says. Of course, it's also likely your gauge will be off unless it is a very high-end unit.

Bob Rouleau

Hey Ron, I got an update, my C4S (my new DD) will arrive in April. Now I feel stupid for buying the snow tires and wheels so fast.
By the way, dealers sometimes use the pressure gauge built into the air hose. Those things are waaaaay off. You'd think the Porsche dealer would have an accurate gauge mind you...!

Best,

axhoaxho

I always wonder this... the TPMS is taking the measure inside the tire, while the tire guage is taking the measure at the valve stem. Can it be both are telling the psi correctly (I don't doubt modern technology on this simple task) but just different readings from different measureing points?

I am no expert on this so it is just my wild guess.... What do you folks think?

With best regards,

russo

My TPMS is off by about 4 lbs. Hence I use the manual gauge for a more accurate reading.

Minok

I thought about that too, and if the tires were filled with liquid, that might be the case, since the valve's nozzel or restriction could increase or change the pressure measured there vs the large interior... but as the tires are filled with gas, the gas law and some basic thermodynamics apply which I think prohibit the pressure exiting the nozzel being different from the pressure in the inside of the tire. Anyone who has not seen at least 23 years of living since they last took a basic chemistry class care to confirm or straighen me out?


My car is a base model, Targa 4, so it came with 18's and I purchased 18's for the winter tires so my door sticker should hold true. The only variation is winter vs summer tires, so to speak. Certainly very easy to adjust my air pressure if all I have to do is bleed some air.... no need to fire up the compressor then.

The manual does say that the PSI readings it gives are NOT to be used for filling but as general guides. but the +- readings at standstill should be used.. which, if I'm to believe them, means I need to knock off 7psi in the back and 2 and 4 respectively in the two fronts.

Dr. Car

My TPMS reads quite low. My recommendation is to get a guage you trust, find out the differential between it and your TPMS readings, and then make the correction in your head when you check pressures on your TPMS.

It's entertaining to see how the pressures go up several pounds when you hit the highway.

Minok

Did you find in using your gauge that the differntial from the recommended pressure for the wheel and what the pressure read at the gauge before adjusting, matched with the differentials reported by the TPMS on its differential screen (the absolute numbers page is never intended to use for adjusting any pressures) ?

RonCT

I think you mean your TPMS (tire press. mon. sys.) reads low. PSM is stability control.

machina

I had a strange but good revelation with the TPMS on my 06 C2S.

One tire always read low by about 3psi and none of them ever agreed with my reference pressure gauge. Last week i got a low warning because i had dropped the pressures for an auto-x. After I set them all to standard street pressure 33/39, now the TPMS is dead on reporting 33/39.

Maybe the system recalibrates after a low warning assuming that you are filling to the specified pressure. Or maybe its just dodgy electrics.

Da Hapa

I don't think the TPMS on my 2008 997S is very accurate at all and do not trust it for checking the correct tire pressures. I do trust it to inform me of a sudden loss of pressure but that's it.

Buy yourself a good tire pressure gauge. It's worth the investment and a much more reliable... well guage

MartyB

My TPS has been telling me for several days now that I have a "Flat Tyre" on the right front. Pressure is fine and no pulling problems. I don't think it can be relied on for anything other than a very sudden change in pressure, and I don't really have evidence that it can be relied upon then.

KeninBlaine

Exactly.

This issue has been discussed at length before. Consensus seems to be as Da Hapa states: use the TPMS to catch any major deviation, but don't rely on the absolute pressures it tells you, as they consistently seem to be different than gauge measurements.

Baggerdude

I agree that the TPMS is not a accurate as a good gauge.

I picked up my TT and the dealer filled the tires with Nitrogen (he says it will keep the tires from flat siding ??) ... whatever.

The TPMS said the cold tires were 32/27 F and 34/35 R.

I took the car out to my compressor and used a gauge Porsche gave me in '89 ... and I use it on all my vehicles. It's good.

The tires were all 'spec' ... even at 20* (F). Thirty minutes later of back road driving and the TPMS was saying different tire pressure for each sneaker .... 2/3 pounds off each set.

The TPMS on the '07 C4S seemed more accurate. But, I'll defer to the gauge.