Use the overall wheel+tire diameter. Should be published with the tire, or you can find it on tirerack.com. Then Pi X diameter to figure out how far the wheel rolls in one turn. Divide 5280 ft by that, and there you go.
Again this is just a rule of thumb sort of thing, but it will give you a good idea.
Or... You could glue a peice metal to the side wall and count the number of whacks in a mile on the interstate...
[This message has been edited by 97snakedriver (edited 04-27-2000).]