Tire Size to Gear Ratio Calculator

Optimize your drivetrain for new tire dimensions

Original Tire Diameter (inches)

New Tire Diameter (inches)

Original Axle Gear Ratio

Recommended Setup

New Required Gear Ratio: –

Current Effective Ratio (with new tires): –

Note: Round up to the nearest commercially available gear set (e.g., if result is 4.21, consider 4.56 or 4.10 depending on use case).

Understanding Tire Size and Gear Ratios

When you install larger tires on your vehicle, you increase the circumference of the wheel. This effectively "stretches" your gears, making your engine work harder to turn the wheels at the same speed. This often results in sluggish acceleration, poor towing performance, and incorrect speedometer readings.

The Physics of Re-Gearing

The goal of re-gearing after a tire size increase is to return the engine to its original operating RPM (Revolutions Per Minute) range. By installing a numerically higher gear ratio (shorter gears), you compensate for the larger diameter of the new tires.

The Mathematical Formula

The calculation to determine the gear ratio needed to maintain your factory performance is:

            New Gear Ratio = (New Tire Diameter / Original Tire Diameter) x Original Gear Ratio
        

Example Calculation

Imagine you have a Jeep with 31-inch stock tires and a 3.73 axle ratio. You decide to upgrade to 35-inch tires. To keep the same engine performance, the math looks like this:

(35 / 31) = 1.129
1.129 x 3.73 = 4.21

In this scenario, you would look for a gear set close to 4.21, such as 4.10 or 4.56 gears.

Effective Gear Ratio vs. Recommended Ratio

If you keep your stock 3.73 gears while running 35-inch tires, your "effective" ratio drops to roughly 3.30. This means your vehicle will feel like it has much taller gears, leading to a loss of torque at the wheels. Re-gearing restores that lost mechanical advantage.

function calculateGearRatio() { var oldTire = parseFloat(document.getElementById("oldTireSize").value); var newTire = parseFloat(document.getElementById("newTireSize").value); var oldRatio = parseFloat(document.getElementById("oldGearRatio").value); var resultsArea = document.getElementById("resultsArea"); if (isNaN(oldTire) || isNaN(newTire) || isNaN(oldRatio) || oldTire <= 0 || newTire <= 0 || oldRatio <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // Calculation for the ratio needed to match stock performance var recommendedRatio = (newTire / oldTire) * oldRatio; // Calculation for what the current ratio "feels like" with bigger tires var effectiveRatio = (oldTire / newTire) * oldRatio; document.getElementById("newRatioResult").innerHTML = recommendedRatio.toFixed(2); document.getElementById("effectiveRatioResult").innerHTML = effectiveRatio.toFixed(2); resultsArea.style.display = "block"; }