Tire Gear Ratio Calculator

Use the formula: Gear Ratio = (RPM × Tire Diameter) / (MPH × 336 × Trans Ratio). For example, if your engine runs at 2,500 RPM at 65 MPH with 30" tires and 1:1 trans ratio: Gear Ratio = (2500 × 30) / (65 × 336 × 1) = 3.43. This tells you your axle ratio is approximately 3.42:1.

Ideal RPM at 70 mph depends on your vehicle and driving style. For fuel economy, aim for 1,800-2,200 RPM. For balanced performance, 2,200-2,600 RPM is common. Most modern vehicles with overdrive transmissions run between 2,000-2,500 RPM at highway speeds. Higher RPM means more power available but lower fuel economy.

Larger tires effectively lower your gear ratio (like having taller gears), reducing RPM at a given speed but also reducing acceleration and power. Smaller tires raise effective gear ratio, increasing RPM. For example, going from 30" to 33" tires with 3.73 gears gives you an effective ratio of 3.39:1—a 9% change that affects speedometer accuracy and performance.

For 35" tires, most enthusiasts recommend 4.10-4.56 gears depending on use. For daily driving with occasional off-road, 4.10 gears provide good balance. For primarily off-road or heavy towing, 4.56 gears restore power lost from larger tires. If you had 3.73 gears with stock 30" tires, you'd need approximately 4.31 gears to maintain the same effective ratio with 35" tires.

For metric tire sizes (e.g., 275/60R17): Diameter = Rim Size + (Section Width × Aspect Ratio × 2 / 25.4). Example: 275/60R17 = 17 + (275 × 0.60 × 2 / 25.4) = 17 + 13 = 30 inches. For flotation sizes (e.g., 35x12.50R17), the first number is the diameter in inches.

Yes. Larger tires cause your speedometer to read slower than actual speed, while smaller tires cause it to read faster. The error percentage equals the percentage change in tire diameter. For example, going from 30" to 33" tires (10% increase) means when your speedometer shows 60 mph, you're actually going 66 mph.