What's the length of a 55 Chevy bel air rear end

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The rear axle length is a hair over 60", the rear wheelwell lips are typically about 71.5-72" across the inside of the uncut stock lips, the inner wheelwell tubs (at trunk floor) are about 46.5" across, making the actual tub width about 12.5" wide. The outside of the springs are about 48" wide and they limit the actual usable width of the stock tubs unless you relocate the springs inboard. The frame is 41.12" wide at the rear and up to the top of the hump, and 42" at the main frame rails. What else do you want to know?

http://www.hotrod.com/how-to/transmission-drivetrain/1205sr-55- 57-chevrolet-rear-axle-housing/