Changing your vehicle's tire size significantly impacts its effective gear ratio, affecting acceleration, towing capability, and highway RPMs. Use this calculator to determine the ideal axle gear ratio for your new tire size, or to predict your engine RPM with a new tire size and your current gearing.
Understanding Tire Size and Gear Ratios
When you modify your vehicle, especially by changing tire sizes, it's crucial to understand how this impacts your drivetrain's gearing. The "gear ratio" refers to the ratio of the number of teeth on the ring gear to the number of teeth on the pinion gear in your differential (axle ratio), combined with the transmission's internal gear ratios. This ratio determines how many times your engine crankshaft rotates for each rotation of your wheels.
How Tire Size Affects Gearing
Larger tires have a greater circumference, meaning they travel further with each rotation. This effectively "raises" your gear ratio, making it numerically lower. For example, if you switch from 30-inch tires to 35-inch tires without changing your axle gears, your engine will spin fewer RPMs at the same road speed. While this might sound good for fuel economy, it also means less torque delivered to the wheels, resulting in slower acceleration, reduced towing capacity, and your engine potentially operating outside its optimal power band.
Why Re-Gear?
Many enthusiasts and truck owners choose to re-gear their axles after installing larger tires for several reasons:
Restore Performance: Larger tires can make your vehicle feel sluggish. Re-gearing restores the original acceleration and responsiveness.
Improve Towing: For vehicles that tow heavy loads, maintaining the correct gear ratio is vital for pulling power and preventing transmission strain.
Off-Road Capability: Lower (numerically higher) gear ratios provide more torque at the wheels, which is essential for crawling over obstacles and navigating challenging terrain.
Maintain Fuel Economy: While larger tires inherently reduce fuel economy, re-gearing to bring RPMs back into the engine's efficient operating range can mitigate some of the losses.
Correct Speedometer: While not directly related to gear ratio, larger tires will throw off your speedometer. Re-gearing often goes hand-in-hand with speedometer recalibration.
The Gear Ratio Formula Explained
The fundamental relationship between engine RPM, vehicle speed, tire diameter, and gear ratios is given by the formula:
RPM = (Speed * Transmission Ratio * Axle Ratio * 336) / Tire Diameter
RPM: Engine Revolutions Per Minute.
Speed: Vehicle speed in Miles Per Hour (MPH).
Transmission Ratio: The gear ratio of the specific transmission gear you are in (e.g., 1.00 for 1:1 direct drive, 0.70 for an overdrive gear).
Axle Ratio: The final drive ratio in your differential (e.g., 3.73, 4.10).
336: This is a constant used to convert units (miles to inches, hours to minutes, and account for the tire's circumference based on its diameter).
Tire Diameter: The height of your tire in inches.
Our calculator uses this formula to solve for the recommended axle ratio or to predict your RPMs based on your inputs.
Using the Calculator
To use the calculator effectively:
Current Tire Diameter: Enter the actual height of your existing tires in inches.
New Tire Diameter: Enter the actual height of the new tires you plan to install.
Desired Engine RPM: This is a critical input. Think about your typical highway cruising speed and what RPM your engine currently runs at, or what RPM you'd like it to run at for optimal power or fuel economy.
Target Cruising Speed (MPH): Your typical highway speed where you want to achieve the desired RPM.
Transmission Gear Ratio: Find the ratio for the specific transmission gear you'll be in at your target cruising speed (usually your highest gear, like 4th, 5th, or 6th gear, or an overdrive gear). This can often be found in your vehicle's owner's manual or online specifications.
Current Axle Ratio: Enter your vehicle's existing axle gear ratio. This is often stamped on the axle housing or found on a build sheet/VIN decoder.
The calculator will then provide a Recommended Axle Ratio to help you achieve your desired RPM with the new tires. It will also show you the Predicted RPM with New Tires & Current Axle Ratio, so you can see the impact of not re-gearing, and the Predicted RPM with New Tires & Recommended Axle Ratio, which should be close to your desired RPM.
Important Considerations
Speedometer Calibration: After changing tire size, your speedometer will read inaccurately. Re-gearing doesn't automatically fix this; you'll need a separate speedometer calibrator.
Transmission Shift Points: Significant changes in effective gear ratio can affect how your automatic transmission shifts. Re-gearing helps restore proper shift points.
Engine Power Band: Ensure your desired RPM keeps your engine within its optimal power band for efficiency and performance.
Cost: Re-gearing can be an expensive modification, involving new ring and pinion gears, bearings, and professional installation.
By carefully considering your tire size and gear ratios, you can ensure your vehicle performs optimally for your specific needs, whether it's daily driving, towing, or off-road adventures.
function calculateGearRatio() {
// Get input values
var currentTireDiameter = parseFloat(document.getElementById('currentTireDiameter').value);
var newTireDiameter = parseFloat(document.getElementById('newTireDiameter').value);
var desiredRPM = parseFloat(document.getElementById('desiredRPM').value);
var targetSpeed = parseFloat(document.getElementById('targetSpeed').value);
var transmissionRatio = parseFloat(document.getElementById('transmissionRatio').value);
var currentAxleRatio = parseFloat(document.getElementById('currentAxleRatio').value);
var resultDiv = document.getElementById('result');
resultDiv.innerHTML = "; // Clear previous results
// Validate inputs
if (isNaN(currentTireDiameter) || currentTireDiameter <= 0 ||
isNaN(newTireDiameter) || newTireDiameter <= 0 ||
isNaN(desiredRPM) || desiredRPM <= 0 ||
isNaN(targetSpeed) || targetSpeed <= 0 ||
isNaN(transmissionRatio) || transmissionRatio <= 0 ||
isNaN(currentAxleRatio) || currentAxleRatio <= 0) {
resultDiv.innerHTML = 'Please enter valid positive numbers for all fields.';
return;
}
// Constant for calculation: 336 is derived from (63360 inches/mile) / (60 minutes/hour * pi)
// However, the standard formula uses 336 directly for MPH, RPM, and Tire Diameter in inches.
var constant = 336;
// 1. Calculate Recommended Axle Ratio for New Tire Diameter
// Formula: RPM = (Speed * Transmission Ratio * Axle Ratio * 336) / Tire Diameter
// Rearranging to solve for Axle Ratio: Axle Ratio = (RPM * Tire Diameter) / (Speed * Transmission Ratio * 336)
var recommendedAxleRatio = (desiredRPM * newTireDiameter) / (targetSpeed * transmissionRatio * constant);
// 2. Calculate Predicted RPM with New Tires and Current Axle Ratio
var predictedRPM_newTires_currentAxle = (targetSpeed * transmissionRatio * currentAxleRatio * constant) / newTireDiameter;
// 3. Calculate Predicted RPM with New Tires and Recommended Axle Ratio (should be close to desiredRPM)
var predictedRPM_newTires_recommendedAxle = (targetSpeed * transmissionRatio * recommendedAxleRatio * constant) / newTireDiameter;
// Display results
var outputHTML = '