Tire pressure

[WardL]

I've been running 22# in my 17" 275/40 in the front and 335/35 in the back. After about 11k miles, I have ever so slightly more tread in the middle of the tires. The tire gauge shows 50 on outside, 58 in the middle and 43 on the inside of the front tires (tread depth). The rears show 53, 56 and 46 respectively. I can't find my tire gauge now but the numbers must be in mm. I think I'm going to bump the pressure up to 28#. I've got some autocross experience showing higher pressure would help. The tires are Michelin PS/2.

[bobcowan]

Originally Posted by genolan

Just took the Cobra for its first real ride since I purchased it in December and checked the tire pressure when done. 38 PSI waited an hour and 36 PSI. The car is a BDR and has the original Goodyear Eagle F1's. What pressure is recommended for this car?

A good rule of thumb is 1 psi per 100# of vehicle weight. For most Roadsters, that's around 24psi.

Originally Posted by WardL

I've been running 22# in my 17" 275/40 in the front and 335/35 in the back. After about 11k miles, I have ever so slightly more tread in the middle of the tires. The tire gauge shows 50 on outside, 58 in the middle and 43 on the inside of the front tires (tread depth). The rears show 53, 56 and 46 respectively. I can't find my tire gauge now but the numbers must be in mm. I think I'm going to bump the pressure up to 28#. I've got some autocross experience showing higher pressure would help. The tires are Michelin PS/2.

Your tire are probably under-inflated. In a perfect world, you'd have even wear across the width of the tread. In the front you'd like to see something like 50-46-43; a nice even spread.

Same thing in the rear. With IRS and a little camber, a closer spread like 53-51-49. If it's a solid axle, it should be a perfectly even spread, like 53-53-53 (in a perfect world).

You need enough pressure in the tire to create a flat contact patch. That's the real answer. How much that is depends on a lot of factors: type and brand of tire, temperature, wheel width, weight of vehicle, etc.

The best way is to check tire temps when the tires are hot. A probe type thermometer is best, but you can use a non-contact infrared thermometer to get you pretty darned close. When hot, stop the car on hot pavement and quickly check outside, middle, and inside of the tread. You gotta move fast, because the surface temp will cool quickly.

When you have camber, the top of the tire leans in. Obviously, that places more weight on the inside of the tire, and that part of the tread gets hotter. So you should see a gradual increase from outside to inside, and no more than 10*.

If there's no camber, like a solid axle, it should be (ideally) the same temp all across the tread.

My car does best with 24 front and 26 rear for street driving.