Tire Diameter to Gear Ratio Calculator
Determine your new effective axle gear ratio when upgrading to tires with a taller overall diameter.
Quick Answer
A truck with 32-inch tires and a factory 3.73 gear ratio that upgrades to 35-inch mud tires will have an effective gear ratio of only 3.41 — sluggish and hard on the transmission. Installing 4.10 gears restores factory acceleration and shift points.
Sizes:
Target Gear Ratios for Taller Tires (3.73 Factory Baseline)
| New Tire Diameter | Factory Gear Ratio | Effective Ratio (Sluggish) | Required New Gear Set |
|---|---|---|---|
| 33 Inches | 3.73 | 3.61 | 3.73 (Acceptable) |
| 34 Inches | 3.73 | 3.51 | 4.10 (Recommended) |
| 35 Inches | 3.73 | 3.41 | 4.10 (Required) |
| 37 Inches | 3.73 | 3.22 | 4.56 or 4.88 (Required) |
| 40 Inches | 3.73 | 2.98 | 5.13 or 5.38 (Required) |
Formula
New Required Gear Ratio = (New Tire Diameter / Old Tire Diameter) × Old Gear Ratio
- Round up to the nearest commercially available gear set (4.10, 4.56, 4.88, 5.13)
- Example: (35" / 32") × 3.73 = 4.07 → round to 4.10
- 3" increase rule — going 3+ inches larger almost always requires regearing
- Both axles must be regeared simultaneously on 4WD / AWD vehicles