Tire & Gear Ratio Calculator
Calculation Results
Current RPM at Target Speed: –
New RPM at Target Speed (with new tires, current axle): –
RPM Change Percentage: –
Required Axle Ratio (to maintain current RPM with new tires): –
Speed at Current RPM (with new tires, current axle): –
Please enter valid positive numbers for all fields.
'; document.getElementById('currentRPM').innerText = '-'; document.getElementById('newRPM').innerText = '-'; document.getElementById('rpmChangePercentage').innerText = '-'; document.getElementById('requiredAxleRatio').innerText = '-'; document.getElementById('speedAtNewRPM').innerText = '-'; return; } // Constant for RPM calculation (MPH, inches, RPM) var conversionConstant = 336; // 1. Calculate Current RPM at Target Speed var currentRPMValue = (targetSpeed * currentAxleRatio * transmissionRatio * conversionConstant) / currentTireDiameter; document.getElementById('currentRPM').innerText = currentRPMValue.toFixed(0) + ' RPM'; // 2. Calculate New RPM at Target Speed (with new tires, current axle) var newRPMValue = (targetSpeed * currentAxleRatio * transmissionRatio * conversionConstant) / newTireDiameter; document.getElementById('newRPM').innerText = newRPMValue.toFixed(0) + ' RPM'; // 3. Calculate RPM Change Percentage var rpmChangePercentageValue = ((newRPMValue – currentRPMValue) / currentRPMValue) * 100; document.getElementById('rpmChangePercentage').innerText = rpmChangePercentageValue.toFixed(2) + '%'; // 4. Calculate Required Axle Ratio (to maintain current RPM with new tires) // Formula: Required Axle Ratio = (Target RPM * New Tire Diameter) / (Target Speed * Transmission Ratio * 336) var requiredAxleRatioValue = (currentRPMValue * newTireDiameter) / (targetSpeed * transmissionRatio * conversionConstant); document.getElementById('requiredAxleRatio').innerText = requiredAxleRatioValue.toFixed(2) + ':1'; // 5. Calculate Speed at Current RPM (with new tires, current axle) // Formula: Speed = (RPM * Tire Diameter) / (Gear Ratio * Transmission Ratio * 336) var speedAtNewRPMValue = (currentRPMValue * newTireDiameter) / (currentAxleRatio * transmissionRatio * conversionConstant); document.getElementById('speedAtNewRPM').innerText = speedAtNewRPMValue.toFixed(1) + ' MPH'; }Understanding Your Vehicle's Tire and Gear Ratios
Modifying your vehicle's tire size or axle gear ratio can significantly impact its performance, fuel economy, and even speedometer accuracy. Whether you're an off-road enthusiast looking for more torque, a performance driver aiming for quicker acceleration, or simply replacing worn-out tires with a different size, understanding how these components interact is crucial. Our Tire & Gear Ratio Calculator helps you predict these changes before you make them.
What is a Gear Ratio?
In simple terms, a gear ratio describes the relationship between the number of teeth on two meshing gears. In a vehicle's drivetrain, the most commonly discussed gear ratio (outside of the transmission itself) is the axle or differential gear ratio. This ratio determines how many times the driveshaft rotates for one rotation of the wheel. For example, a 3.73:1 axle ratio means the driveshaft spins 3.73 times for every one rotation of the wheel.
A "higher" numerical gear ratio (e.g., 4.10:1) means the engine will turn more revolutions for each wheel revolution, providing more torque to the wheels but resulting in higher engine RPM at a given speed. A "lower" numerical gear ratio (e.g., 3.08:1) means fewer engine revolutions per wheel revolution, leading to lower RPM at speed, which can improve fuel economy but reduce acceleration.
How Tire Size Affects Performance
Tire diameter plays a direct role in the effective gear ratio of your vehicle. A larger diameter tire covers more ground with each rotation. If you install larger tires without changing your axle gears, your engine will effectively be "geared taller." This means:
- Lower Engine RPM: At any given speed, your engine will turn fewer revolutions.
- Reduced Torque to the Wheels: The engine will have to work harder to get the vehicle moving and maintain speed, especially uphill or when towing.
- Slower Acceleration: The vehicle will feel sluggish.
- Improved Fuel Economy (potentially): Lower RPMs at cruising speed can sometimes lead to better highway mileage, but if the engine is constantly struggling, it can worsen.
- Speedometer Error: Your speedometer will read lower than your actual speed.
Conversely, smaller diameter tires will make your engine "geared shorter," leading to higher RPMs, increased torque to the wheels, quicker acceleration, and a speedometer that reads higher than your actual speed.
The Relationship: RPM, Speed, Tire Diameter, and Gear Ratio
The core formula connecting these variables is:
Engine RPM = (Vehicle Speed (MPH) * Axle Ratio * Transmission Gear Ratio * 336) / Tire Diameter (inches)
The constant '336' is used to convert units (miles to inches, hours to minutes, etc.) to get a clean RPM value when using MPH and inches for tire diameter.
Using the Calculator
Our calculator allows you to input your current vehicle specifications and then experiment with new tire sizes or target speeds. It will provide you with:
- Current RPM at Target Speed: What your engine is currently doing.
- New RPM at Target Speed: What your engine RPM will be with the new tire size, assuming you keep your current axle ratio.
- RPM Change Percentage: The percentage increase or decrease in RPM, giving you a clear idea of the impact.
- Required Axle Ratio: If you want to install new tires but maintain your original engine RPM at a given speed, this tells you what axle ratio you'll need.
- Speed at Current RPM: This shows you what your new cruising speed would be if you maintained your original engine RPM with the new tires and current axle ratio.
Practical Examples
Let's say you have a Jeep Wrangler with 30-inch tires and 3.73:1 axle gears. You typically cruise at 65 MPH in 4th gear (1:1 transmission ratio).
Scenario 1: Upgrading to 33-inch Tires
Using the calculator with:
- Current Tire Diameter: 30 inches
- New Tire Diameter: 33 inches
- Current Axle Ratio: 3.73
- Target Speed: 65 MPH
- Transmission Gear Ratio: 1.0
The calculator would show:
- Current RPM at 65 MPH: ~2600 RPM
- New RPM at 65 MPH (with 33″ tires): ~2364 RPM
- RPM Change: Approximately -9.08%
- This means your engine will be turning significantly slower, potentially feeling less powerful.
Scenario 2: Finding the Right Gears for 33-inch Tires
If you want to install 33-inch tires but maintain your original ~2600 RPM at 65 MPH, the calculator would suggest a "Required Axle Ratio" of approximately 4.10:1. This indicates that to compensate for the larger tires and bring your RPMs back to a similar range, you'd need to re-gear your axles to 4.10:1.
Important Considerations
- Speedometer Recalibration: Changing tire size will throw off your speedometer and odometer. Most modern vehicles require electronic recalibration.
- Transmission Gearing: Remember that your transmission has its own set of gear ratios. The calculator uses the ratio for the specific gear you select (e.g., 1.0 for direct drive, or an overdrive ratio like 0.7 for highway cruising).
- Engine Powerband: Consider your engine's optimal RPM range for power and efficiency. You want your cruising RPMs to fall within this range.
- Braking and Suspension: Larger tires can also impact braking performance and may require suspension modifications.
Use this calculator as a valuable tool to plan your modifications effectively and ensure your vehicle performs exactly as you intend.