Another wheel backspacing question
I posted a question a while back about wheel backspacing with 255-60-15 tires on my Dart. I have since installed an offset spring kit and I have decided to try and squeeze in a pair of 275-60-15 tires instead of the 255s if I can. I would like some opinions on my wheel backspacing situation before I order 15x8" wheels with custom backspacing. I had a 275-60-15 tire mounted on a 7" wheel with 4 1/4" backspacing (same wheel the 255-60-15 was originally on) which gave me the following dimensions.
With this combination, the inner sidewall of the tire has 1 1/8 of clearance to the leaf spring. The outer sidewall is about 1/16 from the wheel lip. I would like to have at least 3/8 of clearance on both sides of the tire so it appears the same 7 wide wheel with 4 3/4 of backspacing (an increase of ½) would put the tire right where I need it at 9/16 from the wheel lip and 5/8 from the spring. However, being I would like to step up to an 8 wide wheel, I am not sure what backspacing I will need to put the tire in the same position. I have two lines of thinking on this and Im not sure which is correct.
My first line of thinking is that if a 7 wheel with 4 3/4" backspacing puts the tire right where I need it, then to put the tire in the same position in the wheelwell with an 8 wheel all I need to do is take the extra inch of wheel width and divide it in half (1/2 to the back side of the wheel and 1/2" to the front side of the wheel) which would put the backspacing at 5 1/4. Allowing for the extra .4 inches of tire section width due to the extra inch of wheel width (.2 per side), this would put the tire at about 7/16" from the spring and close to 3/8 from the wheel lip.
My second line of thinking is that if 4 3/4" of backspacing on the 7 wheel will work, then an 8 wheel with 4 3/4" of backspacing will keep the wheel hub in the same position as the 7 wheel and will keep the backside of the tire the same distance from the spring (except for the .2 inch increase in section width for the tire now being mounted on an 8 wheel) again putting the tire at about 7/16 from the spring. The front side of the tire will also increase the .2 again putting it at close to 3/8 from the wheel lip.
Which one of these calculations makes more sense to you as it seems either way will get me the same result? Or is there something I am missing that would make for a different dimension on the rear offset?