This is an article worth reading:
http://www.rodandcustommagazine.com/editorial/article.jsp?viewtype=text&id=90138 Ignoring "section width" is one of the most common mistakes when measuring for tires. treadwidth is easy enough but section width is the measurement at the bulge of the tire which can often extend past the rim edge by 1.5" on EACH side.
With the cost of wheels and tires these days you need to double check you measurements about 10 x's and then have someone else measure 10 x's and then if you're lucky find someone with something close that you can "test fit"...
my .02/my experience- I "ate" a pair of rear wheels for my race car because of incorrect backspace ordering....
