Final Drive Ratio Calculator
Cruising RPM from axle ratio, gear and tire size.
Enter your values
ⓘ Live results
Results
- Engine RPM at speed
- 2,047 rpm
- Ideal axle ratio for target
- 3.64:1
- Effective overall ratio
- 2.611:1
- MPH per 1,000 rpm
- 34.2 mph
Quick answer
Engine RPM = (mph × axle ratio × gear ratio × 336) ÷ tire diameter in inches. A numerically higher axle ratio raises RPM and acceleration but hurts economy.
RPM = mph × ratio × gear × 336 ÷ tire diameter
How to use the Final Drive Ratio Calculator
- 1Enter your differential ratio (on the axle tag or build sheet).
- 2Enter the ratio of the gear you cruise in — usually top or overdrive.
- 3Enter tire diameter and the speed you want to check.
Worked example
Using the values the calculator loads with:
Inputs
- Axle (differential) ratio3.73 :1
- Transmission gear ratio0.70 :1
- Tire diameter30 in
- Cruising speed70 mph
- Target cruise RPM2000 rpm
Results
- Engine RPM at speed2,047 rpm
- Ideal axle ratio for target3.64:1
- Effective overall ratio2.611:1
- MPH per 1,000 rpm34.2 mph
Frequently asked questions
Does a higher axle ratio help towing?
Yes — a numerically higher ratio (e.g. 4.10 vs 3.55) multiplies torque at the wheels, which helps towing and acceleration at the cost of higher cruising RPM and fuel use.
Where does 336 come from?
It converts miles per hour and inches into revolutions per minute: 63,360 inches per mile ÷ 60 minutes ÷ π ≈ 336.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.