Equivalent Axle Ratio Calculator
Find the axle ratio that restores stock gearing with larger tires. Compare exact and common ring-and-pinion ratios for 33 to 40 inch tires.
Find ratios preserving original effective gearing.
Equivalent Axle Ratio Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Equivalent Axle Ratio Calculator Works?
The Equivalent Axle Ratio Calculator works by multiplying the original axle ratio by the new tire diameter divided by the original tire diameter. The tool uses 3 steps.
Enter the original setup
Type the factory axle ratio and tire size, for example 3.55 with 265/70 R17 (31.61 in).
Enter the new tire
Type the new size, for example 35x12.50R17.
Read the required ratio
The calculator returns an exact ratio of 3.93 and the nearest common ratios, such as 4.10 or 3.91.
Equivalent Axle Ratio Calculator Formula
The Equivalent Axle Ratio Calculator formula is Ratio_new = Ratio_orig × (OD_new ÷ OD_orig).
R_new = R_orig × OD_new ÷ OD_origE% = (R_chosen − R_new) ÷ R_new × 100- R_orig
- Factory axle ratio
- R_new
- Ratio that restores factory gearing
- R_chosen
- Commercially available ratio picked
Worked example
A truck with 3.55 gears and 31.61 in tires moves to 35 in tires. The required ratio is 3.55 × 35 ÷ 31.61 = 3.93. A 4.10 gear set lands 4.2% shorter than factory, and a 3.91 set lands 0.6% taller.
Equivalent Axle Ratio Calculator Conversion Table
The Equivalent Axle Ratio Calculator conversion table lists the exact axle ratio that restores factory gearing for 6 tire heights and 3 factory ratios.
| New tire height | From 3.21 | From 3.55 | From 3.73 |
|---|---|---|---|
| 32.71 in (831 mm) | 3.32 | 3.67 | 3.86 |
| 33.00 in (838 mm) | 3.35 | 3.71 | 3.89 |
| 34.36 in (873 mm) | 3.49 | 3.86 | 4.05 |
| 35.00 in (889 mm) | 3.55 | 3.93 | 4.13 |
| 37.00 in (940 mm) | 3.76 | 4.16 | 4.37 |
| 40.00 in (1016 mm) | 4.06 | 4.49 | 4.72 |
What is Equivalent Axle Ratio Calculator?
An Equivalent Axle Ratio Calculator is a tool that finds the ring-and-pinion ratio needed to bring gearing back to factory feel after fitting taller tires. The calculation is the reverse of the effective gear ratio calculation.
Axle gears come in fixed steps such as 3.73, 4.10, 4.56 and 4.88. The calculator shows the exact value and the closest available ratios with each one's error.
Builders often pick a ratio slightly shorter than the exact value to regain towing and off-road pulling force.
What are the uses of Equivalent Axle Ratio Calculator?
There are 4 main uses of the Equivalent Axle Ratio Calculator, including regear part selection and budget planning.
Regear part selection
Pick the gear set to order for both axles.
Budget planning
Decide whether a tire upgrade needs a regear.
Front-rear matching
Use one ratio for both axles of a 4WD vehicle.
Highway RPM targets
Pick a ratio that lands cruise RPM in the engine's best range.