Help me confirm my measurements and back spacing thought

70Roadkill said
i have a 1970 Dart Swinger, 8 3/4 rear axle, dr diff big bolt pattern axles, and I installed a 3/4" offset kit.

When I measure from the drum surface to the outer wheelhouse lip it's about 4 7/8" and from the the drum surface to the spring I get 7".

My plan is to is to get a 15x8 rim and put on a set of 275 60 15 tires.

I'm thinking I need a 5" offset to make it work. Can someone chime in to make sure my math is correct.

Thanks

So a 275/60/15 has an 11" section width. Your spring to quarter measurement is 11 7/8". You need a 1/2" to the quarter, which leaves 3/8" to the springs. That's pretty much the minimum clearance you can have and make it work, so, they'll fit but your backspace will have to be perfect.

So, let's get your backspace perfect. A 15x8" is rim is 9" wide outside to outside. The section width is 11", so, 1" of tire hangs over on each side. You've got 7" to the springs, so subtract your 3/8" of clearance, then subtract the inch of rubber, and you've got 5 5/8". That's what your backspace needs to be to clear the springs. Check the math on the front- 3-3/8" front space, plus that other inch of tire, and you get 4 3/8". That leaves your 1/2" to the quarters (which are probably uncut right?).

So, if your measurements are right you should want a 15x8" with about 5 5/8" backspace. Maybe a bit more because it's easier to slide in a thin spacer to get the tires off the springs than it is to cut the quarter lips back. It will be super tight if you don't do any trimming on the quarter lip, but if the math matches the actual section width it should work.