Rear Differential Gear Ratios: Sizing Off-Road Tires and Axles

Upgrading to larger tires is a common modification for off-road vehicles. However, larger tires increase the distance traveled per revolution, which can reduce acceleration and put extra stress on the drivetrain.

Correcting the Gear Ratio

To restore acceleration and engine performance after installing larger tires, you must upgrade to a higher rear differential gear ratio (lower gears). The formula to calculate the ideal new gear ratio is:

$$\text{New Gear Ratio} = \frac{\text{New Tire Diameter}}{\text{Original Tire Diameter}} \times \text{Original Gear Ratio}$$

For example, if your original tires were 31 inches with a 3.73 gear ratio, and you upgrade to 35-inch tires, the ideal new ratio is 4.21 ((35 \div 31 \times 3.73 = 4.21)). A standard gear ratio like 4.10 or 4.56 would be selected to match.

Determine your ideal differential gears using the automotive gear ratio calculator. For detail on ring and pinion dynamics, read our guide on differential ring and pinion mechanics.