How to on axle ratios - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**pjmoreland** · 05-30-2005, 03:50 PM

If you have an accurate tachometer and an accurate speedometer, and either a manual transmission or an automatic with a torque converter clutch, take your truck for a drive and cruise at a constant speed to see what your engine RPM is at that speed. Then you can use the following equation to calculate your gear ratio:

Gear Ratio = Engine RPM x Tire Diameter x .002975 / Speed / Transmission Ratio

If you have a 700R4 Transmission, use a value of .7 for the transmission ratio. If you have a 1:1 high gear, just use a value of 1. If you have an automatic transmission without a torque converter clutch, then this equation will get you close, but the result will be a little higher than your actual gear ratio.

05-30-2005, 03:50 PM	#3
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,321	If you have an accurate tachometer and an accurate speedometer, and either a manual transmission or an automatic with a torque converter clutch, take your truck for a drive and cruise at a constant speed to see what your engine RPM is at that speed. Then you can use the following equation to calculate your gear ratio: Gear Ratio = Engine RPM x Tire Diameter x .002975 / Speed / Transmission Ratio If you have a 700R4 Transmission, use a value of .7 for the transmission ratio. If you have a 1:1 high gear, just use a value of 1. If you have an automatic transmission without a torque converter clutch, then this equation will get you close, but the result will be a little higher than your actual gear ratio.



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