You’ve got questions. We’ve got answers — Mondays, when the Summit Racing tech department tackles your automotive-related conundrums. This week in our Monday Mailbag, we’re looking at the relationship between tire size and engine rpm.
Q: I have a Chevy 350 engine bored and stroked to 383 cubic inches. It has 76cc heads, COMP Cams 280-degree, .480-inch lift cam, Edelbrock Performer RPM intake with a 650 cfm Holley double pumper carb, and a Mallory electronic distributor. The engine is hooked to a Turbo 400 transmission with shift kit. The car is a 1,600-pound T-bucket. With Coker Pro-Trac tires (27 inches tall), what would my rpm be at 60 miles-per-hour with a 4.10 and a 4.56 rear gear axle gear? I have no idea what the gear ratio in the transmission is. Is there a formula for determining engine rpm by tire size?
A: With the 27-inch-tall tires and a 4.10 gear, engine rpm at 60 mph will be about 3,060 rpm. With 4.56 gear at 60 mph, engine rpm will be about 3,400 rpm. The formula to determine this is as follows:
MPH x Axle Ratio x 336 = Cruise RPM at MPH
Cruise RPM ÷ Tire Diameter = Engine RPM
There is also a handy calculator at SummitRacing.com that allows you to plug in variables and determine things like engine rpm, gear ratio, or tire size.