stevepsd

According to the owners manual for my '10 GTS it recommends:

Front - 38psi
Rear - 43psi

or

Comfort Front - 35psi
Comfort Rear - 39psi

However the TPMS display recommends:
Front 34psi
Rear 39psi

(these all are for Partial Loads).

As you can see the TPMS recommendations do not match anything in the owners manual. So, are they all correct?

-steve

Andy E.

According to the door sticker on my '09 GTS, recommended tire pressure is 39/49.
Have you verified that the onboard computer (via the instrument cluster/stalk) has been programmed for 21" summer tires or are you running another size?

Andy E.

Just scanned this from the owner's manual:

Andy E.

Here's the partial load specs:

stevepsd

I forgot to mention that these are for the 255/55-18 All-Season tires that I just bought, not the stock 21's, although the recommended air pressures are the same. I did verify that I had 18" All-Season tires (partial load) selected on the instrument cluster display...which showed 34 front/39 rear as the required pressure, which does not match anything in the owners manual, as the images above show.....hence my slight confusion.

I would have expected the instrument display to show 38 front/43 rear.

My door sticker also shows 39/49 as does your GTS, but that does not match the owners manual either, which shows 39/50 for all size tires, fully loaded....I know it's only 1psi difference, but still....I would not expect this kind of error from Porsche.

-steve

Andy E.

Steve, I totally agree with you.

Slow Guy

Thanks for posting that Andy. I wonder if that's also an (updated) version of the '06 TP's. I run Yoko All Seasons but according to your chart I should drop my pressure back to 35/39 from my current 38/43.

I've had outstanding wear from the Yoko's so far.

I'll see if there's a TSB over on RennTech (I'm sure there is).

mdrums

I'm running Yoko Prada Spec and they really are awesome. I've got 15,000 miles on them now and they are hardly worn. The are quiet, grip well and great in the rain. They are so much better than the Michelin's that came on my Cayenne S.

Usually for me it's just my wife and I in the Cayenne but every week I have to haul around a drumset which weights around 100lbs. I have the OEM 20" Sport Tech wheels. I'm running right between the fully loaded and normal load pressure. Seems to work just fine.

TheUnscented

Bringing this thread back to life. I've got to believe that with a differential in tire pressures like that, we're going to see a slightly different overall tire diameter, circumference and revolutions per mile, front-rear.

Currently I'm trying to match up two tires (failed to rotate regularly, my bad) for my wife's Lexus GS350AWD and the dealer is trying to sell me four tires instead of the worn two. According to the TireRack advice section:

"Four-wheel drive and all-wheel drive vehicles are equipped with additional differentials and/or viscous couplings that are designed to allow momentary differences in wheel speeds when the vehicle turns a corner or temporarily spins a tire. However, if the differentials or viscous couplings are forced to operate 100% of the time because of mismatched tires, they will experience excessive heat and unwarranted wear until they fail.

"Using "matched" tires means all four tires are the same brand, design and tread depth. Mixing tire brands, tread designs and tread depths may cause components in the vehicle's driveline to fail."

This necessitates that four-wheel drive and all-wheel drive vehicles use tires that are very closely matched. This is because different diameter tires roll a different number of times each mile as a result of the variations in their circumferences. Tire diameter variations can be caused by accidentally using different sized tires, tires with different tread designs, tires made by different manufacturers, different inflation pressures or even tires worn to different tread depths.

As an example of different tire diameters resulting from tires worn to different tread depths, we'll compare two 225/45R17-sized tires, a new tire with its original tread depth of 10/32-inch and a second tire worn to 8/32-inch of remaining tread depth. The new 225/45R17-sized tire has a calculated diameter of 24.97", a circumference of 78.44" and will roll 835 times each mile. The same tire worn to 8/32-inch of remaining tread depth is calculated to be 1/8" shorter with a diameter of 24.84", have a circumference of 78.04" and will roll 839 times per mile. While the difference of 1/8" in overall diameter doesn't seem excessive, the resulting 4 revolutions per mile difference can place a continuous strain on the tires and vehicle's driveline. Obviously, the greater the difference in the tires' circumferences, the greater the resulting strain."

Now, how much of this is a scare tactic to sell more tires?

deilenberger

Have you read the transfer case thread? This could certainly be a contributing factor - especially on those cases where the oil was black and smelled burned when it was changed. And it does make a good case for periodic rotation of tires from front to rear and rear to front. Side to side - no.

Rossi

My 2009 GTS has 39 front and 49 rear listed on the doors.

Wisconsin Joe

Some scare, some reality.

In the above example (that I snipped out), the difference amounted to 4 revs per mile.

At 60 mph, that is 4 rpm.

For a clutched system, less is better. Zero is best.

Even for a geared system (like a typical differential), less is better and zero is best.

How worn are the tires you don't want to replace?

If the wear is significant, replacement might be a good idea.

If not too bad, it may be possible to shave down 2 new tires to match.
Or maybe shave them a bit bigger than the older ones an put them on where they wear faster and skip a rotation cycle (Cayennes wear the rears faster, not sure about a Lexus).

Avec

I run the higher pressure up front, generally 47psi cold and 42psi in the back. Hot, these come up to 50 and 45 respectively. I don't care a load or tow, so no need for more pressure in back. Also, the higher front pressure keeps the front end from washing out in tight turns. It makes it handle much more neutrally.

As for the tire diameter difference, it should not be to much of a concern because the car deals with wheel speed differences every time you turn a corner Each of the 4 is going a different speed. This difference has already been accommodated for. A small difference in tire diameter based on wear will not significantly add to this to the point of drive line bind.