Q: My 1974 Chevy Nova has 4.11 gears, but I’m interested in changing them.

I’ll be taking a long trip soon and I’d like to cruise at around 2,000 rpm at 70 miles per hour.

I’m running the original TH-350 with 8.5-inch 10-bolt rear axle, and 28-inch tall tires.

What gear ratio should I use?

I also have installed a B&M 2,400-rpm stall converter. Would this affect my cruising speed? And if I switched to a tire diameter of 26 inches, what would my engine rpms look like at 70 mph?

With my engine putting out around 450 ft.-lbs. of torque, do you think I’d need a transmission cooler for the trip, or will my factory radiator do the trick?

a limited slip rear differential installed with ring and pinion gears
(Image/Jeff Smith)

A:  Here’s a handy formula for you.

Gear Ratio  x  MPH  x  336

­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­———————————————         =        RPM

          Tire Diameter

At 70 mph, your current setup yields: 4.11 x 70 x 336 ÷ 28 = 3,452 rpm.

So, if you use a 2.41 gear, at 70 mph you’ll be at 2,024 rpm. If you switch to a 26-inch tire with the 2.41 gear, you’ll be at 2,108 rpm. Acceleration with this new gear will seem slow compared to your 4.11.

Now, having a 2,400 stall converter means you need to stay above 2,400 rpm at highway speeds or cruising around town.

Some other combinations to consider: a 3.07 gear with your 28-inch tires will let you cruise at 2,528 rpm, while a 2.73 ratio and a 26-inch tire will let you cruise at 2,469 rpm.

With that converter, we’d recommend using a transmission cooler.

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