what the correct tire pressure?

[evol ways]

just noticed my front driver side tire is a little low on air and would like to bump up the pressure but just not sure if i should still use the factory recomended tire pressure on a 255/35/18 tire sitting on 18x9.5+30offset rim so what pressure should i set the tire to? thanks. cars a DD

[CBRicahrd]

There is not just one answer to this question. Think of tire pressures as a dynamic thing, they are constantly changing as you drive your car. Thats why the tire says Cold pressure on the side of it, the pressure will rise after driving on the tire due to friction.

The Factory recommended pressures are what mitsu felt made the vehicle vary safe and stable to drive in all conditions and with all types of drivers. I don't recommend using there pressure specs.

I run 35 cold psi in all four tires, in my daily driven Evo with 245/40r17 tires. I use that spec on the commute to and from work, and also for AutoX. The car feels very neutral and firm. I would bet something real close to that would work just fine for you. You could go lower down to 32 or 30 if you wanted the ride smoother, but it would cause the side wall to flex more during spirited driving. You could go more then 35 if you were looking for a really firm ride and real sharp response.

[Kracka]

Tire pressure all depends on the tire, tire size, the driver, how the cars driven, your other suspension components, etc. You need to figure out for yourself what best suits you, and how making certain changes affects the cars balance.

Here is what I run:
Evo IX GSR w/KYB shocks, Swift Sport springs, Whiteline 24mm rear anti-sway bar in the stiffest setting, 245/45-17 Sumitomo HTRZ III tires
street = 38 psi (front) / 35 psi (rear)
autox = 45 psi (front) / 43 psi (rear)

If I still had the stock rear swaybar I'd run more pressure in the rear to help get the car to have a more neutral feel rather than the stock tendency towards understeer.

[Neal@tirerack.]

Start at oe as that size holds the same amount of air as the 17", just in a different shape. From there tune up to get the feel you desire. Don't exceed the tire max psi listed on the sidewall, don't drop below Mitsu's recommendation.

[Ludikraut]

I run my 265/35 Eagle GS-D3 tires at 38 psi all around for street use. Most lower profile tires will need more pressure than the stock tire. If you look at various cars that come with different tire/wheel combos from the factory, (like Audi and BMW for example) you'll generally see something like 32 psi (+/-2) for something like a 45R17 and 36 (+/-2) for a 35R18. On my wife's R32 I use 32 psi for the 17" winter tires and 38 (per factory recommendation) for the stock 18" tire.

l8r)

[ecuflasher]

Is the recommended tire pressure for 265/35R18 38PSI cold?

[boomn29]

Personally, I always run lower. I've always gotten better wear. Something like 32-35psi cold on everything I drive.

Heck back on my Toyota Celica's (factory Yokohama performance tires) I ran 34 all around and had severely uneven wear down the middle. The next set I ran 32f / 27r - sounded crazy but the rear was sooo light in that hatchback and the tires wore like a dream. 30k miles and still looked to have 1/2 the treadlife left.

[SmikeEvo]

Like Neal said, pressure does not matter on tire size.

I always ran somewhere near 34 front and 36 rear for DD. Then when up or down for AX/Track.

[Neal@tirerack.]

Originally Posted by SmikeEvo

Like Neal said, pressure does not matter on tire size.

Not sure I necessarily said that lol

[Ludikraut]

Here's a good post on the subject, which backs up the data points I illustrated in my earlier post. I don't have the time to do a calculation between the stock tire and a 265/35 right now, though. I'll see about posting that later. This should be enough to get the propeller heads started.

Originally Posted by DamonB @ rx7club.com

...What supports the weight of the car is literally the molecules of air in the tire. Because of this as the volume of air the tire can hold goes up, the required air pressure to support the vehicle weight goes down. Ideally you figure out the exact volume of air inside the stock tire size and that with the inflation pressure would allow you to compute how many molecules of air are inside the tire. Now no matter what size tire you use you merely fill it with the same number of molecules of air. Unfortunately we don't count the number of air molecules when we inflate tires, we measure pressure which is completely different. If you put bicycle tires on your car the pressure would need to be really high, if you put 15" wide slicks on the pressure would be really low.

...

To entertain myself I estimated the volume of air inside your stock 205/60/15 (15" wheels) tires and that inside the 215/45/17 (17" wheels) and found that the 17" tires hold only about 85% of the air volume the 15" tires did. With that in mind then 32psi + 15%= 37 psi. This is merely an estimate. Size has everything to do with pressure. It's not strictly the width of the tire though, it's actually the internal volume of the wheel and tire assembly.

Your new tires are wider so you would expect the air volume to go up, but since they are also on wheels which are much larger in diameter your new setup in fact has less internal volume than your original. That means the required tire pressure will be higher. Start at 37psi and see what happens.

l8r)

[ecuflasher]

So the 265/35R18 tire is wider but holds less volume than the stock 235/45R17 so we will require more pressure than what's specified for the stock 235/45R17?

[Ludikraut]

^ Yep, that's the idea.

l8r)

[Jhero23]

I have two set, I use the same pressure on both. My stock BBS with 255/40r17 RA1 and the other Volk Le37 245/40r18 with star specs. I always go by the hot pressure, 40 psi on all 4.

[kmcconn9]

on my summer tire combo, 255/35 18's, I run 35 up front and 34 out back...

on my winter tire combo, 245/40 17's, I run 33-35 all around....depending on how much grip is needed

[SmikeEvo]

Originally Posted by Neal@tirerack.

Not sure I necessarily said that lol

Ok, implied.