Final Drive Ratio Calculator
Enter a target highway speed, your top-gear transmission ratio, final-drive (differential) ratio, and tire diameter to get your engine RPM — and compare it against a proposed re-gear.
Advanced options (proposed ratio, before/after comparison)
Before / after comparison
How it works
This is the reverse of a typical gear-ratio calculation: instead of asking "how fast am I going at this RPM," it asks "what RPM will I turn at this speed." First, the overall ratio:
Overall ratio = transmission ratio × final-drive ratio
Then the tire's rolling circumference (π × diameter) converts your target speed into wheel RPM, and multiplying by the overall ratio gives engine RPM:
Engine RPM = (speed × 63,360 ÷ 60 ÷ circumference) × overall ratio
Worked example
70 mph, a 1.00 top-gear transmission ratio, a 3.73 final-drive ratio, and a 28 in tire: overall ratio = 3.73, and engine RPM ≈ 3,135. Re-gearing to 4.10 (a common choice after installing larger, heavier tires) raises that to about 3,446 RPM at the same 70 mph — see the before/after table for a full speed range.
Assumptions
- No tire slip, and the tire's actual rolling diameter matches the value entered (not just the sidewall-stamped size).
- Transmission ratio is your highest (likely overdrive or direct) gear, since that's what's engaged at steady highway speed.
- Transfer case ratio is not included here; if your vehicle has one, multiply it into the transmission ratio field first.
Reference
Differential and final-drive gearing fundamentals: Wikipedia — Differential (mechanical device).
Related tools
Frequently asked questions
What does a final drive or differential ratio do?
It's the gear reduction between the driveshaft and wheels. A numerically higher ratio increases torque and RPM at a given speed; a lower ratio does the opposite.
How do I know my engine RPM at highway speed?
Multiply your top-gear ratio by your final-drive ratio for the overall ratio, then convert your target speed to wheel RPM using tire circumference, and multiply by the overall ratio.
Why would someone change their final-drive ratio?
A numerically higher ("shorter") ratio improves acceleration and towing torque at the cost of higher highway RPM; a lower ("taller") ratio does the reverse.