Axle Ratio Tire Size Calculator

Axle Ratio & Tire Size Calculator

Example: 31, 33

Example: 35, 37

Example: 3.73, 4.10

Calculation Results

Required Gear Ratio:

Speedometer Error:

To maintain your current power and RPM levels with larger tires, you should upgrade to the gear ratio shown above.

function calculateAxleRatio() { var oldTire = parseFloat(document.getElementById('oldTire').value); var newTire = parseFloat(document.getElementById('newTire').value); var oldRatio = parseFloat(document.getElementById('oldRatio').value); if (isNaN(oldTire) || isNaN(newTire) || isNaN(oldRatio) || oldTire <= 0 || newTire 0 ? "Under-reading by " : "Over-reading by ") + Math.abs(errorPct).toFixed(1) + "%"; document.getElementById('resultsArea').style.display = 'block'; }

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How the Axle Ratio and Tire Size Calculator Works

Upgrading to larger tires is one of the most popular modifications for off-road enthusiasts and truck owners. However, increasing tire diameter affects your vehicle's effective final drive ratio. When you install larger tires, your engine has to work harder to turn them, which results in a loss of "off-the-line" torque and can cause your transmission to hunt for gears.

This calculator helps you determine the equivalent axle ratio required to bring your vehicle back to its factory performance levels after a tire size change. By matching the ratio to the new tire size, you ensure your RPMs at highway speeds remain in the optimal power band.

The Gear Ratio Formula

The math used in this calculator is a simple linear relationship between tire circumference and gear rotation. The formula is:

New Ratio = (New Tire Diameter / Old Tire Diameter) × Current Gear Ratio

Example: Going from 31″ to 35″ Tires

Let's look at a realistic scenario for a Jeep Wrangler or a light truck:

• Current Tires: 31 inches
• New Tires: 35 inches
• Current Gear Ratio: 3.73

In this case, the calculation would be: (35 / 31) × 3.73 = 4.21. To maintain factory-like performance, the owner should look for a 4.10 or 4.56 gear set, as these are common aftermarket sizes close to the 4.21 target.

Why Re-Gearing Matters

Failure to re-gear after installing significantly larger tires (e.g., jumping from 31s to 37s) can lead to several issues:

1. Transmission Heat: The transmission will slip more or stay in lower gears longer, generating excessive heat that shortens its lifespan.
2. Poor Fuel Economy: While larger tires might theoretically lower RPMs, the engine often falls out of its efficient power band, forcing the driver to use more throttle to maintain speed.
3. Speedometer Inaccuracy: Larger tires travel further per revolution. A 10% increase in tire size means your speedometer will read 60 MPH when you are actually traveling 66 MPH.
4. Loss of Towing Capacity: Your effective torque at the wheels is reduced, making it much harder to pull trailers or climb steep grades.

Common Axle Ratios

Most trucks and SUVs come with factory ratios between 3.21 and 4.10. Common aftermarket performance ratios include:

Tire Size	Recommended Ratio (General)
33 Inches	4.10 or 4.56
35 Inches	4.56 or 4.88
37 Inches	4.88 or 5.13