need help on rear tire and rim size on my 68

Have a 68 Cougar, stock rear , want to run 275/60-15 tires on a 8" rim, anyone know what backspacing I would need to fit without rubbing, or 15 x 7" rim , just want to be able to fit 275/60-15 tires on the rear, running skinnies up front 165/80-15 tires on 15 x 4" rims

This might be of some help.
Believe I found it at the old Cougar website.
WHEELS and TIRES.pdf (252 KB)

275/60/15 is a bit too wide for a 7" rim. You can squeeze it on there but it will bulge the sidewalls some. That tire is better suited for an 8" or a 10" rim. You need at LEAST 4.5" backspace. With a stock width rear, you have as much as 5.75" of space to work with.

Oh and welcome to the group here.

I’m running 255/60/15 on an 8" rim. Clearance isn’t an issue, but if I didn’t have the extra leaf out back, I would be rolling the lip of the wheel well.

275 isn’t going to work on a 7" rim. I have 235 on rear, and 215 on front with 7x15 rims. I (think I would) actually like to move the 235’s to the front and go up to 255 on the rear…I think that’s as big as I would ever want to try with the 7x15’s.

same sizes here, but on 14x6’s in front and 15x8’s (standard Cragar backspacing) in back

has an extra leaf in bach