Example Data Table
| Engine RPM |
Tire Diameter |
Gear |
Axle |
Total Ratio |
Estimated Speed |
| 2500 |
28 in |
1.00 |
3.73 |
3.73 |
55.84 mph |
| 3000 |
31 in |
0.75 |
4.10 |
3.08 |
90.08 mph |
| 2200 |
33 in |
0.68 |
4.56 |
3.10 |
69.58 mph |
Formula Used
Metric tire diameter: Diameter = Rim diameter + 2 × ((Width × Aspect ratio ÷ 100) ÷ 25.4)
Tire circumference: Circumference = Tire diameter × π
Total drive ratio: Total ratio = Transmission gear ratio × Transfer ratio × Axle ratio
Speed from rpm: Speed mph = (Engine rpm × Efficiency × Tire circumference × 60) ÷ (Total ratio × 63360)
RPM from speed: Engine rpm = (Speed mph × 63360 × Total ratio) ÷ (Tire circumference × 60 × Efficiency)
Speedometer correction: Actual speed = Indicated speed × New tire diameter ÷ Old tire diameter
How to Use This Calculator
- Select whether you want speed from rpm or rpm from speed.
- Choose direct tire diameter or enter a metric tire code.
- Enter axle ratio, gear ratio, and transfer ratio.
- Add slip if the drivetrain is not fully locked.
- Enter old tire diameter for speedometer correction.
- Press Calculate to show the result above the form.
- Use CSV or PDF buttons to save the current report.
Understanding axle ratio, tire size, and speed
A drivetrain is a chain of rotating parts. Engine rpm passes through the transmission. It then passes through the transfer case, if one is used. The axle ratio multiplies torque again before the tire turns. Tire diameter turns that rotation into road distance. A small change in any value can move the final speed.
Why this calculator helps
This calculator connects those values in one place. It can estimate road speed from engine rpm. It can also estimate engine rpm from a chosen road speed. That makes it useful for gearing studies, tire upgrades, towing setups, trail builds, and highway cruising checks. The tool also includes slip. Slip helps model torque converter loss, clutch loss, belt loss, or tire growth assumptions.
Physics behind the result
The main idea is circumference. One tire revolution moves the vehicle by one tire circumference. Circumference equals diameter times pi. Wheel rpm multiplied by circumference gives inches per minute. The calculator converts that distance into miles per hour and kilometers per hour. Total drive ratio divides engine rpm before it reaches the wheel. Higher ratios give more torque, but lower road speed at the same rpm.
Using the outputs
The wheel rpm result shows tire rotation. Driveshaft rpm shows rotation before the axle. Total ratio combines transmission gear, transfer ratio, and axle ratio. The effective tire diameter is shown so metric tire sizes and direct tire diameters can be compared. The speedometer correction helps when a vehicle was calibrated for one tire size and then changed to another.
Practical tuning notes
Use measured tire diameter when possible. Loaded diameter can be smaller than the printed size. Air pressure, tire wear, and vehicle weight change the rolling radius. Use gear ratios from trusted vehicle data. For automatic vehicles, lockup status matters. A locked converter has less slip. An unlocked converter may need extra slip. For high speed work, tire growth can also matter. Treat the result as a planning estimate, then confirm it with road data, scan data, or track timing.
Best data practice
Enter values as decimals. Keep units consistent. Save the exported report. It gives a repeatable record for later tire, axle, or gear changes too safely.
FAQs
What does axle ratio mean?
Axle ratio tells how many driveshaft turns create one tire turn. A 3.73 ratio means the driveshaft turns 3.73 times for one wheel revolution.
How does tire size affect speed?
A taller tire travels farther per wheel revolution. At the same engine rpm and gearing, a larger tire usually increases road speed.
Why include transmission gear ratio?
Transmission gear ratio changes how much engine rpm reaches the axle. Overdrive gears reduce rpm. Lower gears increase torque multiplication.
What is transfer ratio?
Transfer ratio applies to vehicles with transfer cases. Use 1.00 for normal high range. Use the low range ratio for crawl calculations.
What does slip percent do?
Slip reduces the effective rpm reaching the tires. It can model converter slip, belt loss, clutch loss, or tire growth assumptions.
Should I use printed or measured tire diameter?
Measured loaded diameter is usually better. Printed tire size may differ because of load, pressure, wear, tread design, and manufacturer variation.
How is speedometer correction calculated?
The calculator compares new tire diameter against old tire diameter. A larger new tire makes actual speed higher than the indicated reading.
Can this be used for racing setups?
Yes. It helps compare gearing, tire diameter, and rpm. For final tuning, confirm results with data logs, track timing, or GPS speed.