Quick answer
Vehicle speed (mph) = rpm × π × tire diameter ÷ (gear ratio × final drive) × 60 ÷ 63,360. At 2,500 rpm in a 3.5 gear with a 3.73 final drive and 27 inch tires, the speed is 15.38 mph (24.75 km/h), with an overall ratio of 13.055.
Enter the gear ratio, final drive, tire diameter and engine speed. The calculator returns the vehicle speed and overall ratio. Use this transmission gear ratio calculator to check your own figures.
Related tools: Gear Ratio Calculator: Gear ratio, output speed and torque multiplication. Gear Speed Calculator: Output speed and ratio of a gear pair. Spring Constant Calculator: Hooke's law spring constant and stored energy.
How to use the Transmission Gear Ratio Calculator
- Enter transmission gear ratio (above 0).
- Enter final drive ratio (above 0).
- Enter tire diameter (in) (above 0).
- Enter engine speed (rpm) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Transmission gear ratio | A number | > 0 |
| Final drive ratio | A number | > 0 |
| Tire diameter | A number, in in | > 0 |
| Engine speed | A number, in rpm | > 0 |
Formula and method
Formula
speed (mph) = rpm × π × tire diameter ÷ (gear ratio × final drive) × 60 ÷ 63,360
IEEE-754 double precision; shown to the decimals indicated.
Ignores tire slip and growth at speed.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Transmission gear ratio = 3.5; Final drive ratio = 3.73; Tire diameter = 27; Engine speed = 2500.
| Result | Value |
|---|---|
| Vehicle speed | 15.38 mph |
| Overall ratio | 13.055 |
| In km/h | 24.75 |
Example 2
Inputs: Transmission gear ratio = 1.0; Final drive ratio = 3.42; Tire diameter = 26; Engine speed = 3000.
| Result | Value |
|---|---|
| Vehicle speed | 67.85 mph |
| Overall ratio | 3.42 |
| In km/h | 109.2 |
Example 3
Inputs: Transmission gear ratio = 0.75; Final drive ratio = 3.08; Tire diameter = 28; Engine speed = 2000.
| Result | Value |
|---|---|
| Vehicle speed | 72.12 mph |
| Overall ratio | 2.31 |
| In km/h | 116.07 |
The first line is the headline result (vehicle speed). Below it the transmission Gear Ratio Calculator lists overall ratio and in km/h, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The overall ratio
Overall ratio is the gear ratio times the final drive: 3.5 × 3.73 = 13.055. One wheel revolution needs 13.055 engine revolutions. A 27 inch tire travels π × 27 = 84.8 inches per revolution.
Use the loaded tire diameter
The loaded diameter is slightly smaller than the unloaded diameter. Tire slip and torque converter slip make real speeds a little lower. For gear changes, compare the rpm at the same speed in each gear.
How the result changes with transmission gear ratio
Holding every other input at the first worked example, this table changes only transmission gear ratio. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Transmission gear ratio | Vehicle speed | Overall ratio | In km/h |
|---|---|---|---|
| 1.75 | 30.76 mph | 6.5275 | 49.51 |
| 3.5 | 15.38 mph | 13.055 | 24.75 |
| 7 | 7.69 mph | 26.11 | 12.38 |
| 14 | 3.85 mph | 52.22 | 6.19 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Transmission gear ratio: entering "0" shows "Transmission gear ratio must be greater than 0." beside that field and no result.
- Engine speed: entering "0" shows "Engine speed must be greater than 0." beside that field and no result.
Choosing between the Transmission Gear Ratio Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Gear Ratio Calculator](/engineering/gear-ratio-calculator/) | driving gear teeth and driven gear teeth | Gear ratio |
| [Gear Speed Calculator](/engineering/gear-speed-calculator/) | driver gear teeth, driven gear teeth and input speed | Output speed |
| [Spring Constant Calculator](/engineering/spring-constant/) | force and extension | Spring constant |
| [Power-to-Weight Ratio Calculator](/engineering/power-to-weight-ratio/) | power, power unit and weight | Power-to-weight |
| [Speedometer Gear Calculator](/engineering/speedometer-gear-calculator/) | teeth on the speedometer drive gear, axle ratio and tire diameter | Driven gear teeth |
Background: structural and wiring estimates
Structural and electrical results are only as good as their assumptions. Bending stress is the bending moment divided by the section modulus, and an allowable stress is the yield strength divided by a safety factor that depends on the governing design code and the type of load. For a single-phase or DC run the voltage drop uses the round-trip length: V = 2 × L × R per unit length × I.
Recommended limits, such as keeping branch-circuit voltage drop around 3%, differ between sources and code editions. These pages are estimates: follow the applicable code and a qualified engineer or electrician for real work.
Assumptions and limitations
- Use the loaded tire diameter
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for transmission gear ratio and engine speed to confirm this.
Frequently asked questions
What is the final drive ratio?
The ratio of the differential, between the driveshaft and the wheels.
How does a taller gear change rpm?
It lowers rpm at a given speed.
Is 63,360 inches in a mile?
Yes, one mile is 63,360 inches (5,280 ft × 12), which converts tire revolutions into miles per hour.
What does the Transmission Gear Ratio Calculator calculate?
Vehicle speed from engine rpm, gear ratio, final drive and tire size. It returns Vehicle speed, Overall ratio and In km/h.
What formula does the Transmission Gear Ratio Calculator use?
speed (mph) = rpm × π × tire diameter ÷ (gear ratio × final drive) × 60 ÷ 63,360. Ignores tire slip and growth at speed.
What do I need to enter in the transmission Gear Ratio Calculator?
You enter Transmission gear ratio, Final drive ratio, Tire diameter (in) and Engine speed (rpm).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Use the loaded tire diameter.
What is an example of a transmission Gear Ratio result?
With transmission gear ratio 3.5, final drive ratio 3.73, tire diameter 27 and engine speed 2500, the vehicle speed is 15.38 mph.
Where does the method come from?
The method follows: OpenStax, University Physics Volume 1 (Rice University): mechanics; 1 mile = 63,360 in (exact; international inch and mile, NIST Special Publication 811 Appendix B).
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Use the loaded tire diameter.
Related calculators
- Gear Ratio Calculator — Gear ratio, output speed and torque multiplication.
- Gear Speed Calculator — Output speed and ratio of a gear pair.
- Spring Constant Calculator — Hooke's law spring constant and stored energy.
- Power-to-Weight Ratio Calculator — Power per unit weight of a vehicle.
- Speedometer Gear Calculator — Number of teeth for the speedometer driven gear after a tire or axle ratio change.
- Belt Length Calculator — Open-belt length for two pulleys.
Sources
- OpenStax, University Physics Volume 1 (Rice University): mechanics
- 1 mile = 63,360 in (exact; international inch and mile, NIST Special Publication 811 Appendix B)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.