Final Drive Ratio Calculator
Cruising RPM from axle ratio, gear and tire size.
Enter your values
Enter the axle (differential) ratio in :1.
Enter the transmission gear ratio in :1.
Enter the tire diameter in in.
Enter the cruising speed in mph.
Enter the target cruise rpm in rpm.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Cruising RPM from axle ratio, gear and tire size. |
|---|---|
| Category | Auto |
| Inputs needed | Axle (differential) ratio, Transmission gear ratio, Tire diameter, Cruising speed, Target cruise RPM |
| Main output | Engine RPM at speed |
| Formula | RPM = mph × ratio × gear × 336 ÷ tire diameter |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Axle (differential) ratio(:1)
- Enter the axle (differential) ratio in :1.
- Transmission gear ratio(:1)
- Enter the transmission gear ratio in :1.
- Tire diameter(in)
- Enter the tire diameter in in.
- Cruising speed(mph)
- Enter the cruising speed in mph.
- Target cruise RPM(rpm)
- Enter the target cruise rpm in rpm.
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.
What do I need to enter into the Final Drive Ratio Calculator?
Just 5 values: axle (differential) ratio, transmission gear ratio, tire diameter, cruising speed, target cruise rpm. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Final Drive Ratio Calculator work out the 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. It applies the formula RPM = mph × ratio × gear × 336 ÷ tire diameter and shows the working so you can check each step by hand.
Is the Final Drive Ratio Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.