924Board.org

[augidog]

I'm running 205/60/15
Firestone firehawks 44 max psi.
What should the pressure be at front and rear?
I've read a lot of differn't answers and I'm confused .
All highway driving.
_________________
1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.

[Smoothie]

[Lizard]

I always run the tires for what they are rated, if you run them at the 29PSI you will probably get underinflation wear on them, I had run a set of them on my 928 and I ran at 40PSI w/o a problem
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3 928s,

[augidog]

I used to run 29 psi, then someone said that tire properties have upgraded since the manual was issued.

I also was told not to run full psi because of heat expansion.

Still confused.
_________________
1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.

[hexen84]

I used to work at a tire shop. We filled most tires up to about 35 psi. That way it had room to expand if needed and when it got cold it wouldn't deflat too much.

[Neil924]

35 psi up front and 36 psi in back, on the 03 Grand am. And for the Potenzas on the 924 32 psi front and back {I think} Thr FireStone I have sitting around has a max pressure of 36 psi, so all cars & tires can be different. Check the specs on the tire and then check the weight of the car in the manual. If your tires start wearing in the middle of the tire, you have too much air. If at the sides of the tread, you see chafing, then you don't have enough air. HTH.

[Smoothie]

I was just going to suggest approx. 35psi all around. That'll give you easier steering and better gas mileage than 29 while still leaving plenty of leeway between under and overinflation. Inflate tires while cold and in the shade and before driving. If you have to drive to an air pump, and want to be real anal about it, put in 36 psi to compensate for the warmed-up tires. Pressures front-to-rear can have some effect on the cars' handling, mainly concerning understeer and oversteer, so go with pressures that you like, but without going to far to either over or underinflation. You can get into all kinds of details like for instance, sidewall flex and if you have different tires front-to-back with different sidewall flex then maybe the flexier ones need more pressure, etc. The best you can do is consider the tradeoffs - gas mileage, turning effort, ride comfort, handling, etc. and go with what you're most comfortable with.
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[ZV]

[augidog]

[ZV]

[Lizard]

I have found that running the tires below the 32PSI mark causes ALOT more sidewall roll in any of the tires that I have driven on, I do however keep my tire pressure the same front and back, and my cars all seem to break free at the same point front and back (which can get alittle interesting) I do run 225 all the way around
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3 928s,

[-nick]

I would concur on starting with ~35psi on a ~44psi max rated tire. Our original sticker is for 20+ year old "radial" tires. Rubber has come a LONG way since then!

Same pressures all around seem to work well for me.

-nick

[emoore924]

How aggressively and under what conditions do you drive your car? That would offer some insight into which combination of traction, ride comfort and even wear might be most important to you.

FWIW, on the street I run old Pirelli P4000 in 205/60/15 and have run about 34-35psi for years. That seems to give me the best combination of traction, ride comfort and even wear. Other tires in other sizes with different carcass consturution would likely require different pressures to exhibit the same characteristics.

The best way to figure out optimal tire pressures is by putting the car on a track, running a half-dozen laps to get some heat into them, then pyroing the inside, middle and outside of the tread. One would then change alignment and tire pressures accordingy. Another way is to "chalk" the sidewalls to check rollover. Completely impractical for street driving of course. I'd probably pay more attention to ride comfort and even wear for the street.

Also, in the case of the 924 I've always paid more attention to what the tire manufacturer says than what the auto manufacturer recommends and here is why: Tire technology has progressed way beyond anything an engineer might have imagined in the late 70's or early 80's when the Porsche recommendation was created. So, although the factory recommendation was probably the "best" at the time, so much has changed with tire technology that I go with what the tire manufacturer recommends... all IMHO.

[ZV]

[augidog]

I'm still am finding online that factory specs are a good starting point.

I found this for guys like Lizard who are running tires out of spec from the optional and standard tire sizes.
Do these calculations work on your car?


"In the event that you aren't able to find a recommendation for your car, or the tires on your car are so different from those that were originally fitted from the manufacturer as to be incomparable, we received the following rule of thumb from Oscar Pereda, an engineer for BFGoodrich. He calls it a "realistic starting point," saying it has never been just right, but is a good place to start. The rule is:

(Vehicle Weight in lb/100) + 2 psi at heavier end + 2 psi all around if suspension and alignment are stock.

Example: Stock 911, 3,000 lb.
(3000/100) = 30 psi
Add 2 psi all around = 32 psi
Add 2 psi to heavy end = 34 psi at rear
With modified suspension, the result is 30 psi front, 32 psi rear."
_________________
1978 924 95 mile daily driver.
Audi TB/POR174M/High Flow Cat/2.25" exhaust
I knew that positive thinking thing wouldn't work.