Wat Rear Gears Are You Running???

[John Poling]

Loaded question, you really need to select a ratio based on the power curve of your engine, the diameter of your rear tires and the final gear (non-overdrive) of your specific transmission. Basically you need to be at your intended cruise RPM and the lower end of your power curve at that RPM. For a typical street use car I have always figured a cruise RPM of around 3000 in your final "NON-O.D." gear. I used to own a 1970 Buick G.S. Stage 1 4-speed, it was a 462ci big block with an M-22 behind it and a 3.64:1 rear gear, cruise RPM in 4th was right at the 3000 RPM point and this was the factory set up.
Now my Cobra is not the large ci variant but I would think that the theory is the same. Keep in mind also that you are not trying to move the mass that a typical big block application would need to. My thought is to error on the conservative side and go with a little lower (numerically) gear maybe a 3.55:1 instead of the 4.56:1.

My current set up is a mild 351W with a C6 (2100 stall) and a 3.55 gear with 315/35/17" tires in the rear which equates to about a 25.75" tall tire. My cruise RPM is just under 3000 RPM at 60 MPH. My drive train was originally built by John York to a 350HP range and pretty much anytime I want a thrilling ride feel it's there with this set up. My basic power curve is from about 1600RPM to 5800RPM.

John