Q&A / Tech

Mailbag: Making Gear Swap Calculations

Got questions?

We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re answering some important questions about swapping gears.

E.B . • Lake Ronkonkonia, NY

Q: I have a 1987 Pontiac Trans Am with a 305 TPI engine, a TH-700R-4 transmission, and 2.73 rear axle gears. I want to upgrade to 3.42 gears, but I want to know if they will be noisy or not. How much of an rpm change will the gears make? I have 225/50-16 tires that are 25 inches tall, and my rpm is 1,600 at 60 miles-per-hour. I know I will also have to change the speedometer gear.

A: Most rear axle gear noise problems can be traced back to improper setup, failure to break in the gears per manufacturer instructions, or using the wrong gear lubricant. Richmond Gear has a good quality lube, and Red Line makes an excellent synthetic gear lube.

As for your rpm change, going from a 2.73 to a 3.42 ratio will increase your rpm at 60 miles-per-hour by approximately 400. You will also need to get a speedometer gear (the driven gear) with four additional teeth to slow the speedometer down by about 20 percent.

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