Cold Tire Pressure

I just want to confirm that regardless of brand and model of tire, the cold pressure should be at the PSI listed on the inside of the fuel door. In my case, 18" 36 PSI front, 44 PSI rear. Correct? Just checked, currently 35 PSI in rear and 31 PSI in fronts.

Mine is listed under the trunk lid.

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So how do I know the proper cold tire pressure? I am running the Aumi's HTR Z3's.

Yes, that's correct, the cold pressure should match what's on the sticker (in my case, it's on the fuel door.). When the tires are hot, it will go up a couple psi and it's almost impossible to do anything consistantly then, so they spec them cold instead. What creates difficulty is that air gauges, especially cheap ones, are notoriously inaccurate. I checked my tire pressures once with 5 gauges I had sitting around (the kids bike pumps, my 12V compressor, a couple pencil gauges, etc) and they had a 15psi deviation! If you can, check your gauge against someone else's. If they read close, they could both be way off, but chances are they're both relatively accurate.

This is a strange question. If you want the tire pressures to match what the sticker says, then you should set them to... what the sticker says. If you want the car to handle its best, under the conditions in which you drive it, a bit more work it required.

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This is a good point I thought probably would get confusing to elaborate on when the question is how to know if your tires match the cold pressures on the sticker. Not to mention a lot of trial and error is required for the best grip with any tire brand/model, car (which corner weights can very considerably with options) and driver, and you'll only notice the difference while driving at the edge of traction, which you shouldn't ever be doing intentionally on the street. As it is, Porsche's recommended tire sizes and pressures create nice, safe, boring, understeer. Which is probably the right way to leave it while off the track, especially for a rear-engine car like this.

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I was just confirming that I was doing everything properly. I then became confused when the mess said that the PSI only applied to "Porsche Approved" tires. Looks like I could be a bit low on the tire pressure. I have a digital gauge and will test again tonight. Then I will make sure that the tires are inflated to 36 PSI and 44 PSI. thanks.

My 2002 Targa was wearing out rear tires in the middle so that they were down to the wear bars at 6-8000 kilometers. Not happy -- it was expensive. So, I went down to 38 on the rears and now the tires wear evenly over the width of the tread -- am now at 16000 kilometers and not half worn -- same brand and type of tire. Porsche dealer checked the alignement -- on all 4 wheels and said -- perfect -- not even a fraction of -- all 4 to within 1% of middle of spec. Where did Porsche get 44 for the rear? Go figure!!!

My INDY (Porsche factory trained and also a former Grand Am Winning crew chief) says the 36/44 for 18" rims is too high and lowered mine last time I was in for service. He also noted that a high PSI on *some* after market alloy's could end up breaking the rims if they hit stuff like potholes and manhole cover lips. He said he had a guy break an HRE a few weeks back and he felt that the 'spec' PSI could have been a factor along with the newer - tougher (but lighter) sidewalls coming on the market pass more impact energy through to the rims now.

just to cloud the issue; on my 2000 996 / sumi tires, i use 1#per 100# of car weight and add 2-4 for the motor. even tire wear so far with at least 3 trac days on them.

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Lowered them to what?

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34/42 My 'plate' says 36/44 and when I asked him about that he says my car was made in 1999 with 1999 era tires. The Conti's I am running are not the orig tires that came with the car (I think they were Pirelli) and tire designs have changed in 10 years. I've been running at the lower settings now for a few months and quite frankly I like the ride far better. He actually suggested I could go as low as 32/40 but not lower then that and not for lapping days, just daily driving. This summer has been very cool and wet around here and the lower settings seems to work quite well as far as grip and confidence.

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That makes 30/34 ? Sounds low for the street no? Good thread. I *just* set mine to factory after driving around for the last few months at 35/42-ish. Don't know if I'll leave it at factory. I'm thinking to air 'em back down to even the tyre wear out.

And, I tend to increase my pressures during the winter since there is a difference between my garage temp and the temp outside. See T/R's tech info: Air Pressure, Temperature Fluctuations http://www.tirerack.com/tires/tirete....jsp?techid=73 Higher Tire Pressures for Winter Driving http://www.tirerack.com/winter/tech/...sp?techid=168& And lastly, Altitude http://www.tirerack.com/tires/tirete...jsp?techid=167

When you post links, please post the actual url. Rennlist now screws with links to certain websites, Tirerack among them.

Oh man, that's Fd up. WTF Fixed

I'm a bit of a maverick when it comes to tire pressures on my '99 C2 coupe. I use the factory lable setting at all times: 36 front, 44 rear. I check them monthly and top them off if they need it. I check them in the morning before I drive the car. I check them monthly and top them off if they need it (note: intentional repeat). I use N-spec tires exclusively, and make certain the build dates match up. I use the same tires for street and track (DE) events. I use the same tire pressures for both. I have the alignment done every year. No uneven wear. Ever. OBC reads over 30MPG on flat sections of highway (Florida, near sea level, 93 octane). At DE's, I run in the Red Run Group (Instructor). When on the track, I don't have any problem keeping up with traffic on street tires, or getting out of the way of Turbo's and race cars, if necessary. I have the alignment done every year (intentional repeat). I replace the rear tires about every 16 months (I do about 6 DE's a year) and the fronts every 32 months. If you drive and maintenance your 996 like a Camry (nee refridgerator-like appliance), you will be disappointed with the results.

those are just a starting point for a generic set of tires. move up/down based on your butt-o-meter. for trac use i would try to keep hot temps in the 40psi range.

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sumitomo=fail

im using kumho exsta xs's at 40 rear 32 front. if you dont know what pressure to run go by the sticker. regardless of where you drive the car you can always check the tire roll with chalk. this will tell you if your pressure is too low if you rub the chalk off the sidewall and too much pressure will not only not rub off chalk but you will lose traction and get center tread wear. (the latter only noticable over time). there is always a few psi leeway up and down from factory psi.

Y'know, this lends itself to one of my all time questions. Pressure recommendations from the vehicle or from the tire. Consensus seems to lean towards taking the pressures from the vehicle but damn....the advances in tire technology give me cause to go with the tire...... Having mused all of that now, methinks a bit lower tire pressures may be warranted, with a close look at wear patterns.

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Wow! Guess it's obvious, but I've never thought much about altitude. I tend to drive at 2k - 4k altitude - and I tend to run 1-2psi below recommended, generally... I get even wear except for an aggressive alignment - I tend to get some wear on the outside with some added neg camber. In essense, I guess I'm actually running 2-4 below recommended, which makes sense that mine equates to. 32-33F and 40-42R Cool - so, I'd chime in w/ my PS2s that 33/41 is optimal... my $0.02 on the topic. Regarding using "sidewall pressures" - now that's just crazy.... isn't that max pressure recommended by the mfgr?

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+1 ...furthermore, tyre manufacturers can't recomend pressures...the weight of the car, the tune of the suspension, and the location of the engine are key variables.

I tried many combinations and settled at 36/38. My current set of rear PS N1's I ran at 36/38 and have over 16k on them. Previous set of rears ran at 36/40 and also got over 16k. At 40cold the rears wore evenly across the tire with the most wear dead center. At 38cold the rears wore most on the inner edge, less on the outer, and the 38 seems a bit more neutra. On ROW 030. Can steer the car with the throttle at AX. -.4/-1.4 camber (mild). Two sets of rears for each set of fronts.

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Unless you are setting your tire pressures at sealevel and then driving to altitude, you are not driving with incorrect pressures. Besides, the adjustment goes the other way. If you set your pressures at 2,000 feet and drive to 4,000 feet then let them cool and check them, the pressures will be too high, not too low. Outside air pressure is less, and thus the air in your tires is relatively at a higher pressure.

More important than most topics IMO. Move on.

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How accurate was the gauge you're using? You could have been inadvertently overinflating above 44psi and making the problem even worse. The short answer to your question, though, is safety and liability. Every tire has a specific pressure for a given load where it offers the best grip. Below this, and it deforms under load, can't effectively press all that tread to the pavement, and grip quickly reduces- in the worst case, you get roll-under. Above this pressure, and it balloons out and grip is lost. Generally speaking, grip is lost quicker at low pressures than high pressures. Wider tires grip better in corners than narrow (it's related to contact patch and slip angle). Case in point is the Boxster- perfect 50/50 weight distribution, so you'd expect the tires to be identical sizes and pressures, right? Instead, Porsche specced out smaller tires in the front. You might think the front tires would therefore need to be at higher pressure than the rear since the contact patch is smaller, and each square inch is carrying more weight. But not Porsche- they again specced out lower pressure in the front than the back that cause the car to understeer like a FWD! Swapping larger tires in the front and pumping the front pressure up reduces the tendency, and gives the car neutral handling. With so much weight in the rear of a 996, Porsche had to go to a lot of effort to keep it facing the right way at the edge of traction. So, they specced out tire sizes and tire pressures that maximize rear grip. If you enter a corner with 44psi in your rear tires, the weight transfer towards the outside will compress them flat and give excellent traction. At lower pressures, the tire will deform and give less traction. In fact, this is what Porsche did in the front- while the rear tires are getting excellent traction, the front tires are deforming under load at stock pressures, and break lose before the rear tires under nearly all conditions, giving you nice, safe, boring understeer. If you raise the front tire pressure, you will find your car performs completely differently at the edge of traction and your back end breaks lose more, simply because you're now able to push the car harder with the better front grip. In short, 44psi = good for car control for the general public, bad for tire wear.

i actually learned something here about deflection and the need for higher psi at high speed= http://www.tirerack.com/tires/tirete....jsp?techid=72

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Thanks for this - I've been asking the question for years but never got a straight answer. One point though..it sounds as though you are saying for AX I should leave the rear pressure at 44 (optimum pressure) but increase the pressure at the front...to 38 or maybe 40. Is that a correct interpretation? Also - what about adjusting pressures when the tyres have some heat in them? If the outer tyre pressure increases because of the extra loading then should I air down to bring it back to optimum?

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FWIW, Boxster weight distribution is ~46/54, IIRC.

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I'm no AX expert, (but I have won my class several times ) but yes, on a stock suspension, increasing your front psi from 36cold and leaving your rears constant, you should reduce understeer somewhat. On my car, with ROW 030, 38/40 cold seems to work well at AX. Very neutral, and can steer with the throttle. However, the suspension tuning is responsible for most all of the reduction in understeer on my car. You may have to experiment with your pressures to find a combination of front/rear pressures that works for your car and your driving style.

I don't profess to be an autocross expert either, but I'm doing well in the local clubs, too. Tire pressure for autocross are something you unfortunately will simply have to use trial and error to find, as different tires require different pressures, and no two cars are ever optioned out/trimmed out exactly alike and thus have different corner weights. Not to mention there's a real amount of preference, too- some people like a lot of oversteer, while others don't. You might even find you need to change the pressure as you progress and your preferences change. I'd recommend scanning through the autocross forum here and try to find someone else running a setup similar to yours, and start there. (Bear in mind anyone on r-comps will be running drastically different pressures, and your gauge is probably reading a couple random psi off from anyone else you talk to.) Chalking your tires will help, too, as optimal inflation pressures are generally around the point where you're using the entire tread and you get back to the pits with just a tiny amount of chalk left above the sidewall. But it will take trial and error to dial in the car from there, and to get it just how you like it. The more experienced guys in your club can help you, too. The best thing you can do is attend a test 'n tune and just play with the pressures, as 3-5 runs per month is going to take a while to dial them in right. Be sure to keep a log, too! One other question: how aggressively do you drive when you autocross? Have you ever spun? Do you frequently hit cones? (If not, you're not driving fast enough, but that's another issue...) If you're an aggressive driver and always on the edge of traction, you'll probably be able to feel 2psi of pressure difference, especially if you adjust the front while keeping the rear at a constant pressure, and vice versa. If you're not yet driving consistantly on the edge of traction, you probably won't be able to notice small differences in tire pressure, and should just pick a decent set of pressures (ie, use the chalk) and stick with it until you improve a bit. And maybe next season, you can give it another go. Speaking of which, bear in mind you *can* put wider than OEM tires on the stock rims.

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Right - I guess I coulda/shoulda clarified...I've been autocrossing for 12 years and was class winner and frequently FTD in the blue 911 you see below. I've only just started autocrossing the 996 though and it is a completely different style of autocrossing. The blue car was very light, very tossable and most of the steering was done with the throttle. The 996 understeers and over the three events I've done and no one could tell me how to tame this. I've got PSS9 coilovers on the way, and of course I'll dial in more camber when I install those, but in the mean time I don't want to be embarrassed by my old air cooled buddies. Yesterday I applied a lesson learned here and increased the front pressure to 38, dropped the rear to 42 and on a rainy day no less, the car handled beautifully neutral with oversteering tendencies. Finally I some decent 911 handling back and I am now placing back in the rankings where I belong. Thanks for the help lads.

hahaha, sorry, the way you asked the question, I just assumed you were a novice. Glad I could be of some help! What organization/class are you racing in? Sway bar settings can help, too.

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We run an autocross series with the local region of the PCA . In the past we tried running proper classes and I ran GTO because of the mods to my car. Then we ran early air vs late air and early water vs late water with and without mods. Nowadays we aren't running classes and the Corvette guys have been coming out so just good times for now. Now that you mention it, I forgot that Iinstalled a set of H&R sway bars a few months ago and this was my first time autocrossing with them on....I wonder if this didn't have as much to do with the improved handling as the tyre pressures Jeezuz I must be getting old! Either way - I'm happy with the setup for now. Cheers!