Proper Tire Pressure?
Safety Car
Joined: Aug 2002
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From: SANTA CRUZ CA
Race Director
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From: Owings Mills, MD 21117
Pro
Joined: Aug 2016
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From: Franklin NC
I do the same on my '96. A little over isn't going to wear out a thing. I like to err a few pounds over vs the wear issues associated with under-inflation.
** I did want to add that my door sticker indicates 30 psi. I'm sure if you have a really accurate tire pressure gauge, 30 is probably perfect if you check it every few weeks.
** I did want to add that my door sticker indicates 30 psi. I'm sure if you have a really accurate tire pressure gauge, 30 is probably perfect if you check it every few weeks.
Last edited by Atomic6; Apr 14, 2017 at 08:18 PM.
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Instructor
Joined: Dec 2016
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From: Toledo Ohio
IM running 40 cold right now, same tires I had on my Firebird, that are sidewall rated 35-52psi without deformation. 45 cold was a nice feel and grip in Firebird but only for; for sure nice summer time driving. Was in the 30s in Vette over winter.
Drifting
Joined: Oct 2015
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From: Shit Creek, USA
You can wear out the center with over inflation, or wear out the edges with under inflation. Or you can go with a digital gauge and the door sticker. On my '96 with stock tire sizes, that works out to 30psi.
It's your money.
It's your money.
Instructor
Joined: Dec 2016
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From: Toledo Ohio
Only if your tires aren't made for it. Dunlop SP Sport 5000 35-52 psi with out deformation, stamped right on the side wall. Door PSI is for the stock tires it came with, if your replacements are built different [ tires are built, not just all rubber ] then following it can be totally wrong.
Drifting
Joined: Oct 2015
Posts: 1,674
Likes: 191
From: Shit Creek, USA
Only if your tires aren't made for it. Dunlop SP Sport 5000 35-52 psi with out deformation, stamped right on the side wall. Door PSI is for the stock tires it came with, if your replacements are built different [ tires are built, not just all rubber ] then following it can be totally wrong.
They didn't have any trouble specifying, in bold terms, the use of Mobile 1 5-30. They could have just as easily printed in the doorjamb 30 psi with Goodyear Gatorbacks ONLY. Alas, they did not. It applies to all tires of the correct size. This even applies to the run-flat tires that came standard on my wifes '95.
But by all means, do what you've gotta do.
Last edited by Renfield; Apr 15, 2017 at 04:56 PM.
Safety Car
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From: Minneapolis Mn
You can always try the old racers trick: chalk. Take some chalk and put a wide mark from sidewall to sidewall across the tread. Go out and drive it a few miles (you may have to go more or less depending)and see how the chalk looks afterwards. It should wear off evenly.
If they're overinflated, there will be more wear-off on the center. If underinflated, more wear-off on the edges. It's fairly fool proof.
If they're overinflated, there will be more wear-off on the center. If underinflated, more wear-off on the edges. It's fairly fool proof.
Race Director
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From: SCMR Rat Pack'r Charter Member..Great Bend KS
We will have to disagree here brother. The engineers measured the contact patch for the weight of the vehicle on the correct size tire - regardless of brand.
They didn't have any trouble specifying, in bold terms, the use of Mobile 1 5-30. They could have just as easily printed in the doorjamb 30 psi with Goodyear Gatorbacks ONLY. Alas, they did not. It applies to all tires of the correct size. This even applies to the run-flat tires that came standard on my wifes '95.
But by all means, do what you've gotta do.
They didn't have any trouble specifying, in bold terms, the use of Mobile 1 5-30. They could have just as easily printed in the doorjamb 30 psi with Goodyear Gatorbacks ONLY. Alas, they did not. It applies to all tires of the correct size. This even applies to the run-flat tires that came standard on my wifes '95.
But by all means, do what you've gotta do.
I mistakenly put 35psi in my tires one morning before a spirited mountain run with the local CF members. I had a hard time keeping the rear end behind me. In fact, at one point I ended up not catching the rear end in time, which resulted in a 360 degree spin. (caught on camera by the car behind me). I couldn't figure out why the car was so loose until I checked the air pressure right after checking my shorts. After I bled the tire pressure back to 30psi, the car stuck and handled normally.
More air pressure means a reduced contact patch area = less traction.
Be careful playing around with tire pressures on the street....and don't make the mistake of thinking if 30 is good, 34 or 35 is better. It sometimes is worse, much worse.
Last edited by rocco16; Apr 15, 2017 at 08:00 PM.
Safety Car
Joined: Aug 2013
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From: Minneapolis Mn
Remember that unless you maintain the size of rims and tires on your car as the mfg put on them, you might have the need to make adjustments to your air pressure to compensate. Another thing is the age of the tires, they can harden over time affecting handling so it may be necessary to make adjustments because of that also.
The door sticker may be a good starting place, but it may not be the final value that's needed in every situation.
The door sticker may be a good starting place, but it may not be the final value that's needed in every situation.
Last edited by hcbph; Apr 16, 2017 at 07:16 AM.
Race Director
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From: SCMR Rat Pack'r Charter Member..Great Bend KS
Only two things affect contact patch size: corner weight and tire air pressure. Tire size, neither bead diameter, section width nor section heght, has nothing to do with contact patch area.
Unless you are a very experienced autocrosser trying to fine tune handling characteristics, or are on a NASCAR pit crew, I wouldn't futz around with tire pressure. Go with the factory recommendations and be safe. Experimenting on the track is one thing; the streets can be unforgiving.
Trying to outguess the design engineers will almost always result in a bad choice.
Last edited by rocco16; Apr 18, 2017 at 04:33 AM.
Instructor
Joined: Dec 2016
Posts: 169
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From: Toledo Ohio
We will have to disagree here brother. The engineers measured the contact patch for the weight of the vehicle on the correct size tire - regardless of brand.
They didn't have any trouble specifying, in bold terms, the use of Mobile 1 5-30. They could have just as easily printed in the doorjamb 30 psi with Goodyear Gatorbacks ONLY. Alas, they did not. It applies to all tires of the correct size. This even applies to the run-flat tires that came standard on my wifes '95.
But by all means, do what you've gotta do.
They didn't have any trouble specifying, in bold terms, the use of Mobile 1 5-30. They could have just as easily printed in the doorjamb 30 psi with Goodyear Gatorbacks ONLY. Alas, they did not. It applies to all tires of the correct size. This even applies to the run-flat tires that came standard on my wifes '95.
But by all means, do what you've gotta do.
Safety Car
Joined: Aug 2013
Posts: 4,430
Likes: 609
From: Minneapolis Mn
Its only valid if you run the exact same brand and type of tire the car came with. Every tire is made different, performs different, reacts different to a said PSI setting . 30 psi in one tires, is not 30psi in another tire, said contact patch will not be the same just cause the PSI is. Soon as you change brand and type of tire and or size, the door sticker is 100% wrong, you then have to defer to the tire manufacturer on what is best.
I'll give an example. I have 2 sets of tires for my truck, winter and summer in the same size but one set are P ratings and the other E ratings. The P's are run at 55 psi while the E's are at 65 psi to maintain the proper contact pattern while in use. They've both been run over 50K each with normal tread wear on both sets.
I just changed from 255x50zr16 tires to 255x40zr18 tires on the Vette. I've only put maybe 100-200 miles on the new ones but at 34 psi they're sloshy and the car wants to weave a lot. Upped the pressure to 38 & 40 front and rear and it handles a lot better. I have not yet chalked the tires but that's on my list of things to do and once done any final adjustments will be done then to insure full tread contact is maintained.
I agree that the factory values may be a good starting point but it's not guaranteed that it's the best value for your final results.
Last edited by hcbph; Apr 17, 2017 at 06:33 AM.