There are three main criteria in measuring a wheel. You need to know the:
1) TIRE SIZE - the tire size is located on the side wall of the tire
2) PILOT - the diameter of the center hole of the wheel
3) BOLT PATTERN - determine the number of lugs on the rim, then measure the distance from one hole to the space directly across the pilot hole

     If there is an even number of lugs, measure from the center of one bolt to the center of the bolt directly across the pilot hole.

     If there is an odd number of lugs, measure from the center of one bolt to the space directly across the pilot hole.

Example: If there are 4 lug holes and the distance from the center of one bolt to the center of the bolt directly across the pilot hole is 4" then the bolt pattern is 4 on 4.