Originally Posted by pakmanpony
My new (new to me) 2008 Unlimited came to me with new Cooper Discoverer R/T3 LT285/70R17 tires instead of the stock P255/75R17 tires.
What range should I be looking at for the tire pressure? Stock tires were at 35psi.
Also, how does this affect my speedometer reading in MPH? Do any of you have your speedo adjusted when you change tire diameter?
You should run 33's at around 30-32 PSI. A lot of tire shops, dealers, etc overinflate em, usually going by the door placard. The 30-32 is just a starting point. To find out the best PSI for any tire, use the chalk test.
There are two versions:
For the first, you simply draw a straight line across your tires, then drive on em and see how the line itself wears. You can use chalk, grease pencil, crayon, etc...just make sure it's a dry day. The line should be worn away evenly across the tread of your tires...if there's too much chalk left in the middle, the tire is overinflated. Too much left on the sides, underinflated.
There are some photos at the bottom of this link to illustrate what I mean:
For the second, you need some sidewalk chalk and a flat asphalt parking lot or driveway. Here's a link:
Your speedo accuracy won't be affected by the change in tire pressure; only if you change tire size.