Gear Ratio Tire Size Calculator

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Gear Ratio & Tire Size Calculator

function calculateGearRatioTireSize() { var currentTireDiameter = parseFloat(document.getElementById('currentTireDiameter').value); var newTireDiameter = parseFloat(document.getElementById('newTireDiameter').value); var currentAxleRatio = parseFloat(document.getElementById('currentAxleRatio').value); var transmissionRatio = parseFloat(document.getElementById('transmissionRatio').value); var targetSpeed = parseFloat(document.getElementById('targetSpeed').value); var resultDiv = document.getElementById('result'); resultDiv.innerHTML = "; // Clear previous results // Input validation if (isNaN(currentTireDiameter) || currentTireDiameter <= 0 || isNaN(newTireDiameter) || newTireDiameter <= 0 || isNaN(currentAxleRatio) || currentAxleRatio <= 0 || isNaN(transmissionRatio) || transmissionRatio <= 0 || isNaN(targetSpeed) || targetSpeed <= 0) { resultDiv.innerHTML = 'Please enter valid positive numbers for all fields.'; return; } // Calculation for Effective New Gear Ratio // If new tires are larger, effective gear ratio decreases (numerically smaller) // If new tires are smaller, effective gear ratio increases (numerically larger) var effectiveNewGearRatio = currentAxleRatio * (currentTireDiameter / newTireDiameter); // Calculation for RPM // RPM = (Speed_MPH * Axle_Ratio * Transmission_Ratio * 336) / Tire_Diameter_Inches var constant = 336; // Conversion constant for RPM calculation var currentRPM = (targetSpeed * currentAxleRatio * transmissionRatio * constant) / currentTireDiameter; var newRPM = (targetSpeed * currentAxleRatio * transmissionRatio * constant) / newTireDiameter; // Display results var resultsHTML = '

Calculation Results:

'; resultsHTML += 'Effective Axle Gear Ratio with New Tires: ' + effectiveNewGearRatio.toFixed(2) + "; resultsHTML += 'RPM at ' + targetSpeed + ' MPH with Current Tires: ' + currentRPM.toFixed(0) + ' RPM'; resultsHTML += 'RPM at ' + targetSpeed + ' MPH with New Tires: ' + newRPM.toFixed(0) + ' RPM'; // Add comparison if (newRPM > currentRPM) { resultsHTML += 'With the new, smaller tires, your engine RPM at ' + targetSpeed + ' MPH will be higher.'; } else if (newRPM < currentRPM) { resultsHTML += 'With the new, larger tires, your engine RPM at ' + targetSpeed + ' MPH will be lower.'; } else { resultsHTML += 'Your RPM at ' + targetSpeed + ' MPH remains the same with the new tires.'; } resultDiv.innerHTML = resultsHTML; } .calculator-container { font-family: Arial, sans-serif; max-width: 600px; margin: 20px auto; padding: 20px; border: 1px solid #ccc; border-radius: 8px; background-color: #f9f9f9; } .calculator-container h2 { text-align: center; color: #333; margin-bottom: 20px; } .form-group { margin-bottom: 15px; } .form-group label { display: block; margin-bottom: 5px; font-weight: bold; color: #555; } .form-group input[type="number"] { width: calc(100% – 22px); padding: 10px; border: 1px solid #ddd; border-radius: 4px; box-sizing: border-box; } button { display: block; width: 100%; padding: 12px; background-color: #007bff; color: white; border: none; border-radius: 4px; font-size: 16px; cursor: pointer; transition: background-color 0.3s ease; } button:hover { background-color: #0056b3; } .result-container { margin-top: 20px; padding: 15px; border: 1px solid #e0e0e0; border-radius: 4px; background-color: #e9ecef; color: #333; } .result-container h3 { color: #007bff; margin-top: 0; } .result-container p { margin-bottom: 8px; line-height: 1.5; }

Understanding how your vehicle's gear ratio interacts with tire size is crucial for optimizing performance, fuel economy, and even speedometer accuracy. This calculator helps you predict the impact of changing tire sizes on your effective gear ratio and engine RPM at a given speed.

What is Gear Ratio?

A gear ratio describes the relationship between the number of rotations of an input gear to the number of rotations of an output gear. In a vehicle's differential (axle), it's the ratio of the number of rotations the driveshaft makes to the number of rotations the wheels make. For example, a 4.10 gear ratio means the driveshaft spins 4.10 times for every one rotation of the wheel. A higher (numerically larger) gear ratio provides more torque multiplication, leading to quicker acceleration but typically lower top speed and higher RPMs at cruising speeds. A lower (numerically smaller) gear ratio offers less torque, slower acceleration, but potentially better fuel economy and lower RPMs at cruising speeds.

Why Tire Size Matters

Your vehicle's speedometer, odometer, and effective gear ratio are all calibrated based on the factory-installed tire diameter. When you change the diameter of your tires, you alter the number of rotations the tire makes to cover a certain distance. Larger tires cover more ground per rotation, while smaller tires cover less.

How Tire Size Affects Effective Gear Ratio and RPM

When you install larger diameter tires, your vehicle's engine has to work harder to turn them. This effectively "lowers" your gear ratio (makes it numerically smaller), meaning less torque is delivered to the ground for the same engine RPM. Conversely, installing smaller diameter tires effectively "raises" your gear ratio (makes it numerically larger), providing more torque but potentially higher RPMs at cruising speeds.

The engine's RPM (Revolutions Per Minute) at a given speed is directly influenced by the tire diameter and the overall gear ratio (axle ratio multiplied by transmission gear ratio). Larger tires will result in lower RPMs at a given speed, while smaller tires will result in higher RPMs.

Using the Calculator

To use the calculator, you'll need a few pieces of information:

  • Current Tire Diameter (inches): Measure your existing tires from the ground to the top, or find the specification online.
  • New Tire Diameter (inches): The diameter of the tires you plan to install.
  • Current Axle Gear Ratio: This can often be found on a sticker in your glove box, door jamb, or by checking your vehicle's specifications online. Common ratios include 3.08, 3.73, 4.10, 4.56, etc.
  • Transmission Gear Ratio: This is the ratio of the specific gear you are interested in (e.g., 1.0 for a 1:1 direct drive gear, or 0.7 for an overdrive gear). You can find this in your vehicle's transmission specifications.
  • Target Speed (MPH): The speed at which you want to compare RPMs (e.g., highway cruising speed).

Example Scenario:

Let's say you have a truck with 30-inch tires and a 4.10 axle gear ratio. You're considering upgrading to 33-inch tires for better ground clearance. You want to know how this will affect your highway RPM at 65 MPH in a 1:1 transmission gear (ratio 1.0).

  • Current Tire Diameter: 30 inches
  • New Tire Diameter: 33 inches
  • Current Axle Gear Ratio: 4.10
  • Transmission Gear Ratio: 1.0
  • Target Speed: 65 MPH

Using the calculator, you would find:

  • Effective Axle Gear Ratio with New Tires: Approximately 3.73 (4.10 * (30 / 33))
  • RPM at 65 MPH with Current Tires: Approximately 2981 RPM
  • RPM at 65 MPH with New Tires: Approximately 2710 RPM

This example shows that by going to larger 33-inch tires, your effective gear ratio becomes numerically lower (like having 3.73 gears instead of 4.10), and your engine RPM at 65 MPH drops by about 271 RPM. This could lead to slightly better fuel economy on the highway but might reduce acceleration performance. Use this tool to make informed decisions about your vehicle's setup!

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