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: Here’s a handy formula for you.
Gear Ratio x MPH x 336
——————————————— = RPM
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.