Looking to put new wheels and tires on a 1977 Cutlass supreme. Can anyone tell me if 255 60 15 tires will fit on the rear of this car and should the rims be 15 x 8 or will that tire fit on a 15 x 7 rim?

 

I ran 255-60s on 15x7 wheels on my car for a long time, so the tires will definitely fit on the wheels, but I can't say how much clearance there would be on your vehicle. Also consider that the wheel backspacing/offset makes a big difference on how much clearance there will be.