Originally Posted by grayfish
FYI: Your measurement does not work for 5 bolt patterns.
I guess I should have been more specific... the measurements on a 5 lug pattern will never be exact, because of there being an odd number of studs. You are correct in that. In those instances, though, the measurements are only SLIGHTLY off. Lets say the wheel is positioned like this (jeep wheel, 5x4.5" BP):
The distance from the top stud to either one of the lower studs is about 4.380", or just over 4 3/8". A hair under 4.5". Since there are no other measurements that are closer than 4.5" on a BP, then it is assumed it's a 5x4.5" bolt pattern. Or you can measure from the back of one to the center of the other. All works out the same.
EDIT: for picture