Q&A

# Mailbag: Optimizing Gear Ratio for RPM at Cruising Speeds

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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

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.

1. Thomas says:

3.55 gear 700r4 with 2000 stall lockup should do nice

2. Joe Sendelbach says:

Run a 200 R4 tranny and it would likely be like having a 2.73 gear in the rear end at hwy speeds and take off like its got a 4.56 rear.

3. John SERENO says:

I plan to go with “GEAR VENDORS” overdrive. Check it out – very strong. You would be able to keep what you have – gears, tires. You change tail section of driveshaft with this unit, some electronics and shorten driveshaft?. Their site tells you more and costs about \$2K. Not real cheap but when you add cost of gears, installation, tires, etc not much more. It something you can do yourself.

4. Allen says:

I think some of the guys with Turbo 400 in C4 where doing this, it gave them over drive.

5. Scott says:

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

Need to? Why? I can’t believe this lie is still being spread in 2019. When cruising, you’re putting far less than maximum engine torque into the converter, since you don’t need that much for steady speed driving, so the effective stall will be far less than the full rated stall RPM of the converter in any given combination. In other words, at part-throttle, the converter will be basically “locked up” long before you reach maximum stall RPM.

When you’re writing for an audience of people trying to learn, please stop spreading mis-information and do a little more research first.

For the record, I have over 10,000 miles of street driving with my current setup, using a 4800 stall converter with a 4.11 rear and 28″ tall tires, using overdrive on the freeway to cruise at 60+ MPH at 2700 RPM, with zero issues.

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