I'm running a MkII on 15" wheels with Cooper Cobra GT Radials's sized: P245x60 R15/P255x60 R15 Front/Rear with all at 34 lbs. I can't seem to find anything about optimal tire pressure on the Cooper site nor by searching the forum. Suggestions?
Personally, I think that is way too high. These cars are so light, it doesn't take as much pressure as a heavier car to obtain proper tread pattern.
A lot has to do with how wide your rims are, also.
Aside from performance and handling, the object is to set them to a pressure that will result in even treadwear all the way across. Too high a pressure will wear them out in the center of the tread pattern, too little pressure will wear them out on the outside edges more and create too much heat as the sidewalls flex excessively during operation.
This can be a trial & error process. Sometimes you can get an idea by starting at a higher pressure and letting out a few pounds at a time and seeing how the tire lays down a pattern or picks up something like baby powder from the floor.
In the beginning they will only pick up the powder in the center of the tread pattern. As you let more air out of the tires and roll them across the baby powder, you will see that they pick up more powder toward the outside of the tread, until you get to the point where it reaches the outside edges.
Mine is a MKII, with BFG Radial TAs up front in 235/60R15 at 21 psi and the rears are 275/50R15 BFG Drag Radials at 17 psi.
I hope that helps you. Who knows, maybe I'm making this more complicated than it needs to be!
Thanks guys. I'll calibrate using the baby powder. It seems to makes sense to empirically lower the pressure to the point where the full width of the tread sits evenly on pavement. I'll post the results as it may help someone else using the Coopers.
porxter is exactly right. For street vehicles, you want to optimize treadwear, and that depends primarily on the weight of the vehicle. The way you drive the car can have an effect on treadwear patterns, too, but unless you're nuts (like me) and drive too fast all the time, it shouldn't be a problem. I always use a tread depth gauge and measure all the way across each tire to make sure I'm getting even wear. Whenever I get a new car, it usually takes me about half of the life of the tires before I get the pressures nailed down, but after that, it's a done deal.
As a side note, when I picked up my '07 Jeep Grand Cherokee SRT8, it was all over the road, especially during high speed lane changes. Turns out the dealer lot boys put 44 psi in all the tires!!! 34/32 front/rear is the right pressure for it. It sticks pretty good for a 4800 pound beast!
Keep experimenting and adjusting, and you'll get it just right before too long.
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