Equivalent Axle Ratio Calculator
Find the axle ratio that brings back stock gearing. Use the calculator below. Results update as you type.
How the Equivalent Axle Ratio Calculator Works
This calculator works by finding the axle ratio that restores factory effective gearing after a tire upsize. It then picks the nearest ratio sold for that axle.
- Enter the factory axle ratio, for example 3.73.
- Enter the old tire diameter, for example 31 in.
- Enter the new tire diameter, for example 35 in.
- Read the exact ratio needed (4.21) and the nearest standard ratio (4.30).
The Formula
The formula is Required Ratio = Factory Ratio × (New Diameter ÷ Old Diameter).
Required Ratio = R_factory × (D_new ÷ D_old) Nearest Ratio = closest of 3.08, 3.23, 3.42, 3.55, 3.73, 3.92, 4.10, 4.30, 4.56, 4.88, 5.13, 5.38Worked example: a 3.55 axle with 30 in tires changed to 33 in needs 3.55 × 33 ÷ 30 = 3.90. The nearest sold ratio is 3.92.
Available ratios depend on the axle model. Check the gear maker's list for your axle.
Conversion Table
The conversion table gives the exact and nearest standard axle ratio for 10 upsizes. Exact ratios run from 3.63 to 5.21.
When the exact ratio sits between 2 standard ratios, the higher one gives more launch force and the lower one gives lower cruise RPM.
| Factory Ratio | Tire Change | Exact Ratio Needed | Nearest Standard Ratio |
|---|---|---|---|
| 3.55 | 30 → 33 in | 3.90 | 3.92 |
| 3.73 | 30 → 33 in | 4.10 | 4.10 |
| 3.73 | 31 → 35 in | 4.21 | 4.30 |
| 3.92 | 31 → 35 in | 4.43 | 4.30 |
| 4.10 | 32 → 35 in | 4.48 | 4.56 |
| 4.10 | 33 → 37 in | 4.60 | 4.56 |
| 3.73 | 31 → 37 in | 4.45 | 4.56 |
| 3.42 | 33 → 35 in | 3.63 | 3.55 |
| 4.56 | 35 → 37 in | 4.82 | 4.88 |
| 4.56 | 35 → 40 in | 5.21 | 5.13 |
What Is the Equivalent Axle Ratio Calculator?
The Equivalent Axle Ratio Calculator is a tool that finds the ring-and-pinion ratio that brings back factory gearing after fitting larger tires. Re-gearing is the standard fix for sluggish acceleration after a large upsize.
The calculator works from the effective gear ratio. If the factory effective ratio was 3.73 with 31 in tires, a 35 in tire needs 4.21 to match it.
Common Uses
This calculator has 4 uses: re-gear planning, parts ordering, front-rear matching and budget planning.
- Plan a re-gear: Find the target ratio.
- Order parts: Pick the nearest sold ratio.
- Match front and rear axles: Order the same ratio for both on 4WD.
- Plan budgets: Decide between re-gearing now or later.