Effective Gear Ratio Calculator
See how tire size changes your gearing. Use the calculator below. Results update as you type.
Effective Gear Ratio = Original Ratio × (Old Diameter / New Diameter)How the Effective Gear Ratio Calculator Works
This calculator works by scaling the axle ratio by the old tire diameter divided by the new one. A bigger tire lowers the effective ratio, which feels like taller gearing.
- Enter the axle ratio, for example 3.73.
- Enter the old tire diameter, for example 31 in (787 mm).
- Enter the new tire diameter, for example 35 in (889 mm).
- Read the effective ratio: 3.30.
The Formula
The formula is Effective Gear Ratio = Original Ratio × (Old Diameter ÷ New Diameter).
Effective Gear Ratio = Original Ratio × (D_old ÷ D_new) Change (%) = (D_old ÷ D_new − 1) × 100Worked example: a 4.10 axle with 33 in tires changed to 37 in tires becomes 4.10 × 33 ÷ 37 = 3.66. That is a 10.8% drop in effective ratio.
Conversion Table
The conversion table shows the effective value of 3 common axle ratios (3.73, 4.10 and 4.56) after 10 tire changes. Changes run from 28 in to 40 in tires.
A 2 in upsize on a 30–35 in tire lowers the effective ratio by about 6%. A 4 in upsize lowers it by about 11%.
| Old Tire | New Tire | 3.73 becomes | 4.10 becomes | 4.56 becomes |
|---|---|---|---|---|
| 28 in | 30 in | 3.48 | 3.83 | 4.26 |
| 29 in | 31 in | 3.49 | 3.84 | 4.27 |
| 30 in | 32 in | 3.50 | 3.84 | 4.27 |
| 31 in | 33 in | 3.50 | 3.85 | 4.28 |
| 31 in | 35 in | 3.30 | 3.63 | 4.04 |
| 33 in | 35 in | 3.52 | 3.87 | 4.30 |
| 33 in | 37 in | 3.33 | 3.66 | 4.07 |
| 35 in | 37 in | 3.53 | 3.88 | 4.31 |
| 35 in | 40 in | 3.26 | 3.59 | 3.99 |
| 37 in | 40 in | 3.45 | 3.79 | 4.22 |
What Is the Effective Gear Ratio Calculator?
The Effective Gear Ratio Calculator is a tool that shows how a tire size change alters the vehicle's real gearing. The axle ratio stays the same, but the distance covered per axle turn grows with tire size.
Lower effective gearing means less wheel force, slower acceleration, lower engine RPM at cruise and more time in higher gears. Owners of lifted trucks use the figure to decide whether to re-gear.
Common Uses
This calculator has 4 uses: re-gear decisions, acceleration prediction, towing checks and transmission tuning.
- Decide on re-gearing: See how far the effective ratio dropped.
- Predict acceleration: Estimate the loss in launch force.
- Check towing: Compare effective gearing with towing guidance.
- Tune transmissions: Adjust shift points for the new gearing.