Ron V

I need some suggestions for tire pressure. I'm running 225/45 r 17s in the front, and 255/40 r17 in the rear on cup 2 wheels. The tires are Kumho ECSTA ASX. I'll be driving normal highway to semi agressive moutain roads. 88 S4 auto trans. Any help would be great, thanks.

dr bob

Ron-

I run the same sizes, but Michelin PS2's. 38 in front, 36 in rear. The PS2's are sensitive to lower pressures, evidenced by rolling the outer rib and scrubbing into the sidewall a little. I notice it by the time I get down to 34 in the fronts. I don't know how the Kumhos compare when it comes to tread and sidewall stiffness. You can do a little chalk/tire crayon test with marks from the first rib down onto the sidewall a bit. Then do a few aggressive corners and see how much you scrub off the sidewalls below the tread. Don't want it to get past the tread bock edges ever, that's for sure.

On the rears, I monitor wear pretty diligently. Looking for a telltale center area wear problem that says pressure is too high. I also monitor the fronts for uneven wear. looking for signs of alignment issues like inner band wear telltale that says the toe is drifting out.

I have a tire tread depth gauge that indicates in 64/ths. Need to add a good IR thermometer to the tool collection for instant tread temp reads. Between those two and a good tire pressure gauge, you can tell an awful lot about what they need.

Marine Blue

I like the recommended pressures noted on the gas cover.

Tahoe Shark

Timely question Ron! Like Dr Bob I would suggest 38/36 to start (cold) with but by the time you get up here you will have gained about 4psi in the cold pressures. Let some air out on Sat morning. You know we don't drive agressively up here. Just follow the Beast and you will be fine.

Ron V

Thanks All. Who said agressive?

deutschmick

I have Michelin PS2's with the same wheels and tire sizes that you're running on my '91GT. I usually use 36F and 42R filled with N2 and it seems to work well. Dr. Bob's response is more scientific though. Good luck!

mark kibort

with this gas crisis!!!!! 100 MEEEEELION PSI

I just pumped up my BMW street tires to about 40psi and im getting about 2mpg better now!

MK

dr bob

More is better, right up to the point where the 928 rides on a narrow strip in the middle of the rear tires. I remember casually observing the rears on several 928's well before I bought mine, and thinking how dumb it was to have so much rear pressure, to the point where the rear tire wear was sp noticeably uneven. On the front PS2's, I could easily run 40-42 PSI, but the ride and street handling both suffer a lot. The 38F and 36R I run now are definitely a compromise for wear, handling, and ride quality. I've been a preacher for the basic tenet that rear press is always higher than front. But a few comments from Heinrich provoked me to try the slightly lower rear press, and the results were confirmed with the crayon and the tread depth gauge. I think I mentioned somewhere else that I replaced one front (due to bruise or mfr'ing defect causing a sidewall bubble...) with a little over 6k on it, with less than 20% wear. Puts me on track to get 25-30k tire life, not a bad range to be in with real performance tires.

SharkSkin

I second Bob's recommendation to figure it out by experimenting. You might be surprised at what pressures give you an even contact patch, evidenced by even temp across the tread. Chalk/crayon and an IR gun, combined with driving under the conditions for which you want to optimize tire pressure, will tell you the optimum "hot" pressure. Once you figure this out, let the car sit overnight and check again to find the "cold" pressure to use(assuming no leaks).