jjborgesi

Hi All, I am trying to figure out what rear end gears to use for a 1971 cutlass convertible with a 455 and M20 four speed. The car will be used for cruising not racing? Thanks for the replies

Miles71

I have seen some old specifications for 455 and axles.
Some Highway cars would come with 2.5x and 2.7x ratios from the factory.

Most Cutlass Supreme with 350's came with 2.7x ratios.
My 1970 has a factory 2.78, Anti-Spin (Posi Traction).
My 1971 has a factory 2.73?, 1-wheel drive.

With a 2.7x ratio, you will be turning about 2,100 RPM at 65 MPH.

bccan

My 2 cents-
I'd put a 3.23 in it, would pull nice w/ M20 and not grate on you @ highway speeds. 3.42 would be great too but starts pushin many people's buttons on interstate.

Fun71

I agree.

Rear gear selection depends upon rear tire size, too. Tall tires can make low gears more highway friendly.

anthonyP

With the 455 and higher numerical first gear in the M-20, my opinion would be a 3.08 rear axle ratio. Should provide plenty of power to quickly get to cruising speed, while still providing a nice livable rpm while cruising.