I have a 68-72 12 bolt to go into my 65 malibu. Before I go and waste my money on rims and tires, I need to know what back spacing I will need on the back and front. I would like to run 15x8 on the back and 15x7,15x6, etc. on the front. Maybe 275/60s on the back with something a little smaller on the front. Maybe a lesson on meassuring back spacing is in order, anyways any help will be appreciated.

The 68-72 12 bolt will be 1-1/4" wider than the stock rearend. You need to mount it before trying to make the backspacing measurements. I measured and found that 5" of bs for 8" wide wheel works perfect for a stock axle. This centered the wheel in the wheelhouse. A 7" wheel fits best with a 4-1/2" bs for the front. I have 8" on all four corners and believe the front should be a 7" with the 4-1/2" bs. I have minor rubbing on bumps with the wheel turned in a corner. I have lowered the front 1" and installed a 72 PB setup.

To measure your setup, install the 12 bolt axle first. Support the car under the rear axle with jack stands with the wheels off. Place a piece of flat stock against the face of the brake hub and measure the distance from the edge to the inner wheelhouse wall and outer wheel lip. Measure it at the front edge, a couple of middle places, and the rear edge of the wheel lip. Then graph the wheelhouse cross section out using the narrowest width of one set of the measurements made and place the centerline of the axle facing on this grpah. Next draw an 8" wide wheel box centered in this wheelhouse graph. Then measure the distance from the back edge of the wheel box to the axle centerline for the proper bs required.

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Brian TC#1092, ACES#4374
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1965 El Camino (http://www.chevelles.com/showroom/65elcamino1b.jpg) Before Restoration (http://www.chevelles.com/showroom/65elcamino1a.jpg)
1981 Corvette (http://www.chevelles.com/showroom/65elcamino81.jpg)

Great post, thanks.....