Tyre and Rimsize/back space

[Ian1960]

I have a 69 Mach1and would like to fit larger tyres on the rear and would like to know rimsize, backspace and tyre width. I do have a size in mind and that is 275 on either a 15" or 17" rim.

 

[Mach1 Driver]

Hi Ian, Barnett468 is a restorer who has done more than 50+ of these cars. He says that the largest tire that will fit on the front without hitting slightly in some conditions is P235/60-R15. It is 26.1" diameter and 9.3" wide, 15x7 wheel, 4.25 backspace.
It will of course depend on what you do to the suspension, adding lots of variables. Following is a post from another website that gives good details, and he uses the classic Magnum 500 wheels- I love the look of this car:

 

[KBMWRS]

Note that 15" tires are becoming harder to find these days.

 

[Horseplay]

Use whatever reference as simply a guide. No two mustangs are EVER exactly alike. Hell, you can buy two sets of the exact same manufacturers springs and after installation/driving find measured differences on the same car. I suggest taking very detailed measurements in all critical clearance areas with an existing wheel/tire and then do the math to see how much space might be available. Remembering, of course, that rears do shift slightly when driving. Suspensions do travel, etc. If you have cars buds, see if anyone has a mounted tire of the size you suspect wanting and borrow it for a test fit. Use math to adjust the fit if the current wheel offset, etc is not what is needed. OR make/buy a tool that mounts to the axle and mimics the dimension of the wheel/tire you want. Bottomline, don't just trust others as to what they say will fit!