Evan opined in news: snipped-for-privacy@posting.google.com:
I just bought a 66 mustang with a 5.0 engine and a t-5 transmission.
> It has 15" rims on it now, but I would like to put 17x8s on. I would
> like to know what backspacing to get so that I will be able to put the
> biggest tires possible on.
>
> -Evan
>
Here's how I did a rough measurement on mine..
Raise the car so it's level.. but in axle being measured, place stand under control arm or axle housing as close to wheel as possible to get hub at ride height.
Use straight-edge to locate centerline in wheel-well, On back wheel, measure across to seam.
On front, take width of tire, not wheel, divide by 2 check for clearance of fender lip and inner fender.
Assume you are going for close to same height overall?