Some time ago when visiting tire sites like Tire Rack, I noted they measured offsets from the INSIDE of the tire bead surface to the flange face, I guess to be consistent with measuring width, use the same plane....
but for ME, I found it much easier to calculate just laying a 2x4 over a naked rim on the outside of the tire flange on the flat portion, not that little turn out lip....and so, by my measure off the stock steel '72 wheels I found it was 3.5" offset 15x8 rims....
the '89 vette rims are 17x9.5" and measured 7" even....so with my 2.5" adaptor they net out at 4.5"
since they obviously were 1.5" wider overall, that is 3/4 inches per side, with regard to the wheel center line....makes my wheel off the stock center by 1/4 inch, hell with it, wanted 275 rubber to clear the stock t-arms, and moved the ebrake cable to behind the rotor, most guys seem to think the top position is better, but for some reason I chose behind the rotor, don't remember why...been 15 years now...
so my tires clear the stock t-arms by about 1/4 inch, and are really close to the stock 7/16? sway bar...
my car was a old custom that at one time had wicked flares, and so were trimmed down to something strongly resembling a later shark, at the time the burgundy paint was done by previous owners.....I had a friend here with a bone stock '72 and his flares were much less then mine, but yet mine were just about the same as another friend with a '78.....also a bone stock body...
I have no issues with tires sticking out, or rubbing....
