Need help with rim back spacing?

There is another thread in the "Tires & Rims" section that discusses the backspace topic in detail. I could not immediately locate it.

Per previous advice I have read at this site, I will probably just take a straight edge and hold it directly against the wheel hub. From there, measure straight in (toward the middle of the car) throughout the entire range in which the wheel will turn.

The narrowest measurement is the one to work with. (ie: leaf spring for rear wheels). You have to allow space for clearance from the sidewall of the tire; this is up to your comfort level and I have seen some people with very negligible spacing. You also have to factor in the section width of the tire you want to use; that is given with the tire specifications. The leftover amount should be the allowable backspacing.

On the front, your primary concern will be contact with the upper control arm, both running straight and when you turn the steering wheel.

If you want to purchase something specifically for the purpose of checking backspacing and wheel diameters, Summit Racing has the following for $70.

http://www.summitracing.com/search/?keyword=PHP-01201&dds=1