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 don’t have much room for error. Currently I have 15”x7” 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 I’m 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?

Take some measurements with your current wheel and tire setup, then play with your options at this website to see what the differences are.

http://www.wheelsmaster.com/rt_specs.jsp

Seems like you are dealing with a 66-67 B-body? Not sure but if that is true, your numbers are right by plan. I've got 7in wheels witn 4.5 backspacing on 275/60/15 tires with 5/8 in on both sides. I think that, by your numbers, you will need 8 in with 5 in back spacing to fit evenly. Problem is little or no production wheels are that dimension. That is last time I looked. Also going from 7 to 8 inch rim will net a negligable change in section width. Just my 2 cents here. I believe that you are looking at this correctly. Good luck

My car that I am trying to figure this out for is a Dart. I got the .2" increase/decrease in tire section width per 1/2" change in wheel width from one of the tire manufacturer websites. Can you explain how you determined that I need 5" of backspacing on the 8" wheel?

not sure what style of rim you want, but the older centerlines are 8.5" wide and have ~5" backspacking, might be just what you need. They fit great with 30x10 slicks on my 69 Coronet, though that does you little good.

this is why I prefer talking offset vs. backspace. offset is measured from the wheel centerline, backspace, from the inside edge of the wheel.

a typical 7" rim is 8" overall width, an 8" rim is 9" OAW (because the width is measured bead to bead)

so you found you need a 7" rim with 4 3/4" backspace. that would be 3/4" positive offset. so you'd need an 8" wheel with 3/4" positive offset. that would be an 8" rim with 5 1/4" backspace. if you get an 8" rim with the same backspace as the 7" rim, it'll only be 1/4" positive offset, and ALL of the extra 1" width will go to the outside.