This gear ratio calculator shows engine RPM at any speed in each gear, from your transmission, axle ratio and tire diameter. Pick a transmission to load its published gear ratios, then adjust the axle ratio, tire size and speed.
The gear ratio formula
- Engine RPM = MPH × axle ratio × transmission gear ratio × 336.13 ÷ tire diameter (inches)
- Speed (MPH) = RPM × tire diameter ÷ (axle ratio × transmission gear ratio × 336.13)
The constant 336.13 converts inches per minute of tire travel into miles per hour (63,360 inches in a mile ÷ 60 minutes ÷ π). Example: a 4L60E in overdrive (0.70) with 3.73 gears and 31-inch tires at 65 mph turns 65 × 3.73 × 0.70 × 336.13 ÷ 31 = about 1,840 RPM.
The result ignores torque converter slip, so actual RPM is slightly higher until the converter locks up. Measure tire diameter, or use the diameter from the tire size, because worn or low-pressure tires are smaller than new.
Why change gear ratios?
- Bigger tires lower the effective gearing, which can make a truck feel sluggish and keep the transmission hunting for gears; a numerically higher axle ratio restores it.
- Towing benefits from lower (numerically higher) gearing for pulling power.
- Highway cruising benefits from overdrive and a numerically lower axle ratio for lower RPM.
Every transmission’s ratios are listed in our transmission gear ratios chart. Shop replacement units by transmission code or by make: Chevy and GMC · Ford · Dodge and Ram.
Shipping is a flat $80.00 per order to US addresses, with 15-day free returns. Not sure which one fits? Ask a parts specialist.