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Before we get started, here is the heart of the parts being installed from Chevs of the 40's. They have a 3.55:1 ring-and-pinion gear set for '37-50 Chevy trucks that help bring cruising rpm down to a more tolerable level. Most half-ton trucks have 4.11:1 gears and a 1:1 final drive, by switching to the 3.55:1s you'll drop about 500 rpm at 65 mph. If you have 4.57:1 gears, it'll drop by about 800 rpm.