Need Wheel Measurement Help Please


Nov 2, 2019
Cross-posting this from a Reddit post I submitted...

I've been working on a disc brake conversion as part of my 68 coupe restoration. This week I installed my new Ford 9" axle . I'm about to order a set of wheels for all 4 corners since the disc brakes require a larger wheel diameter. I've measured everything repeatedly and mocked up the wheels several different ways. Still, as a final sanity check, I'm hoping someone here can help me triple check my measurements.

I'm aiming for a set of Torq Thrust 17x9.5s in the rear. The measurements are as follows:

TT Wheel overall width: 10.5"

Quarter panels lip to lip: 69.125"

Wheel mounting surface to mounting surface (@ rotors): 60"

Spring to spring (outside edge, and same as inner well to inner well): 45.5"

Assume a 255/40R17 Nitto NT555 G2 which measures 10.24" wide, 25" diameter.

The wheels have options for 6.35 and 7.06 backspace (28mm and 46mm offsets respectively).

If I did my math right, a 6.35" backspace puts the tire's inner edge at 1.03" away from the spring and 0.54" away from the quarter panel lip. That's a bit too tight on the outside edge for my taste. I'd like at least 0.75"

With 7.06" backspace, it should put the tire 0.32" from the springs and 1.25" from the quarter panel lip. With a 1/4" or 3/8" shim, I think that that puts the wheel right in the sweet spot.

1/4" shim should give 1" outside and 0.57" inside

3/8" shim should give 0.88" outside and 0.695 inside

I think that's correct, and it seems to check out with my mockup, but I would love a second opinion! 7" is a lot of backspace and not everyone likes shims (longer lugs shouldn't be necessary with shims below 1/2"). Still, I tend to follow Mike Maier's line of reasoning on cheating backspace and shimming for a perfect fit. Thanks for the help!
  • Sponsors (?)

The rear axle can move an inch sideways when you slam the gas real hard
If the tires rub too hard you'll burn up your clutch or automatic trans
Try to use no shims
Roll the fender lips like my Boss 302 has (fairly easy)