For a "crude" measurement, measure from the CENTER of one stud to the FAR EDGE of a stud two studs away. Round that to the nearest quarter of an inch and that would be your bolt circle diameter. I have measured (with a tape measure) 5-1/4" center to center of two studs that are two studs away from each other. Add the 1/4" for half the diameter of one of the studs and you're at 5-1/2" bolt pattern. It's not anywhere near real exact, but if you have a tape measure at a swap meet, you can buy those wheels with confidence!