You will need to go to 33's to get into that range. Typically actual diameter is and inch to an inch-and-a-half SMALLER than the nominal dimension given. i.e. 31x10.50R15 range somewhere between 29.5 and 30.5 inches in diameter. LT versions of the tires get a little closer to the nominal mark but not by much. Nominal tire dimensions are NOT actual inflated diameter. Everything from tread design and depth to inflation pressures will affect the actual diameter of a tire once its on the vehicle.

Most manufacturers will list RPM (Revolutions Per MILE) in the tire specs. You can calculate exact speed from that.

Keep in mind too, that changing tire diameter that much is GOING to affect your passing and climbing power if you leave the stock gears in the axles. Take it from someone who ran 33's in a 92 with 3.55:1 axles and a 302. Turned the Bronco into a gelding!

 

Yep....been thru it in the past. This one looks terrible with the 30x10.50x15's on it now. I figure to get a tire as close to a true 32" tall and it should be ok in both looks and go power.

If I have a real need for speed, the '11 GT/CS handles that just fine.