I did a search on various topics and didn't quite find an answer to what I was looking for.

I have a 2007 CS with 18" wheels. Obviously I have TPMS. On page 299 of the manual is states that the wheels should have a cold air pressure of 29 psi for the front and 26 psi for the rear.

On the drivers side door sill, it states the cold tire pressure should be 32 psi for the front and 37 psi for the rear.

I filled my car's tires accourding to the pressure suggested on the door sill and now, according to the TPMS, the rear tires are fine, but it states the front tires have +2 psi in them.

Now I know that there is going to be a difference +/- 1 psi on the tire gauge and the TPSM, but my question is which should I follow? The manual, the door sill or the TPSM? Or should I ignore all of them and just fill my tires according to my external, hand held tire gauge?

Thanks in advance for the input.

Yes the manual and placard are different. I'd go with the placard since it has to pass government approval. The TPMS will always give a different reading than a normal air guage because of the different measurment process. Use a quality air guage for filling and checking the tires and the TPMS as an early warning system to alert you of an impending problem.

I put this in another post: "Porsche uses a "direct measurement system" made by Beru which has a pressure sensor in each tire which broadcast a signal to an antenna mounted in each wheel well that is further transmitted to a central processor. This has been found to be superior to the less expensive "indirect measurement system" that uses the relative speeds of each tire as calcuated from the ABS system. For more see http://www.beru.com/download/produkt...pekt_tss_e.pdf

The system itself never really knows what the actual "relative" pressure is - meaning, the differential between outside (ambient) temperature, and pressure inside the tire. The sensor is only INSIDE the tire. When you check the tire pressure the old-fashioned way, with a gauge on the valve stem, you are reading relative pressure - the differential between the pressure in the tire and the pressure outside the tire. The gauge reads zero before you put it on the tire, it does not read 13.5 PSI, which is the absolute air pressure of the atmosphere surrounding the tire.

TPMS is a warning system that can save tires, wheels and lives however tires should be routinely checked while cold with a tire pressure guage like the Intercomp that Brad recommends (knocks my others in the garbage can).

Thanks for the quick response. I was going with what was on the car as well, but I also didn't want to deviate from the manual, hence the question. I tend to check my air pressure before a hard drive and at every fill up.

Thanks again,

Scott

I had the same question. Good to know.

Yeah, my car came from the dealer with tires filled to 29 front and 36 rear.

After a bit of driving they'd rise to about 30 front and 37 rear.

On really cold mornings (like today) the fronts were down to 27 and the rear down to 34.

I'd actually kind of like to turn the TPMS off if it starts to act up at all. I've driven for 16 years. I can feel a flat tire (I've had 2). And I check my tires once a week so I'll catch an underinflated tire pretty quick. The TPMS may start to be nothing more than a nussiance if it starts acting up. Hopefully it will be fine.

it seems to me that your fronts are underinflated. [edit - not sure what tire size you are running, but that is too low for my 235/35-19s]

regarding tpms versus being able to sense a flat with your arse, one thing to consider is that you likely won't be able to conclusively discern a tire at only 15psi that is overheating and potentially ready to fail. good to know at 85mph.

i was told by the dealer that my tpms system was malfunctioning when the car was delivered to me. they even reset it and told me that it would display a warning for the first several miles and they sent me off. turned out the dummies had given the car to me with less than 25 psi in the front tires. there was absolutely nothing wrong with the system.

__________________
- chris

'07 987CS
'71 911E

I have the same set-up as you... 18" wheels with TPMS. My door sill says 30 front 37 rear. When I inflate to these pressures (as measured on a manual tire gauge), the TPMS says everything is exactly right. So I'd say lower the fronts to 30psi and you'll be fine.

Use a manual gauge to get the pressures right. The TPMS doesn't necessarily show the same readings as the manual gauge... the driver info readout tends to show 27/34 when cold... but when you enter the TPMS menu they show as being the correct pressures (+0).

By the way, I did try playing with pressures a bit, and found that even 1 psi over on the fronts produced much more tramlining (following road imperfections). So, I make sure to keep the tires at the recommended 30/37 psi. (18" rims only... the recommended pressure for 19's is 32 front).

Hope this helps.

You make an interesting point. When I bought the car, it had 19" rims on it and I switched them out for 18" rims. (which would explain the discrepancy on the door sills to an extent but not with the manual) Perhaps the TPMS needs to be reset. I have checked the info screen on the TPMS and it states I'm running 18" summer tyres (which I am), do I need to take it to the dealer to have something else reset?

You must set the TPMS on board computer to the size wheel and summer/winter tires. It is easy to do but does require the computer to re-learn with a few miles of driving. Sounds like yours has been re-set so all is o.k.