back spacing and sizes

I remember the guys that ran 15x10's on the back of their Dusters. Not a good look unless you mini tub and stuff them up inside. Back then a pair of air shocks would get you there. Not a good plan.

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Yup, that’s about how you have to do 15x10’s if you’re going to run enough tire to actually use the 10” rim.

With a 275 you might be able to stuff it into the quarter, but you don’t need 15x10’s for 275’s either. Just easier to run the 15x8” they’re supposed to be mounted on.
If I can find 15x 9 wheels what would a could back space be for the rear setup ? And would they be too much for the front ? probably would be but might be good option for rear , seeing some places that will produce these sizes

They’d work in the rear, you’d need a 5” backspace to make it work. Again, it really doesn’t buy you anything. A 275 is the widest tire you can run with the stock spring locations unless you hang them out of the quarters. And a 275 is perfectly happy on a 15x8”.

Not happening in the front. Even 15x8’s don’t work in the front. You can’t run more than 4.5” backspace because the rim hits the brakes/suspension. Even 4.5” is pretty marginal for brake/suspension clearance. And with a 15x8” that’s not enough backspace to get the wheel under the fender with any width tire that’s worth running the 8” rim. With 4.25 to 4.5” of backspace on a 15” rim the most you’re going to fit on most of these cars is a 225, which fits easily on a 15x7”.