I could use some help in determining what backspacing I need to order on an 8 wheel. I am working with limited wheelwell space and dont have much room for error. Currently I have 15x7 wheels with 4 1/4" backspacing and 275-60-15 tires mounted on them. 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 1/2") would put the tire right where I need it at 9/16 from the wheel lip and 5/8 from the spring. However, I would like to step up to an 8 wide wheel but 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 also 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 this would put the back side of the tire at about 7/16 from the spring and the front side of the tire at close to 3/8 from the wheel lip.
Do either one of these lines of thinking make more sense to you as it seems either way will get the same result? Or do you have another idea on what the correct backspacing on the 8" wheel should be?
