.gear-ratio-calculator-container {
font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
max-width: 700px;
margin: 20px auto;
padding: 25px;
border: 1px solid #e0e0e0;
border-radius: 8px;
background-color: #ffffff;
box-shadow: 0 4px 12px rgba(0, 0, 0, 0.05);
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.gear-ratio-calculator-container h2 {
color: #2c3e50;
text-align: center;
margin-bottom: 25px;
font-size: 26px;
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.gear-ratio-calculator-container .calc-input-group {
margin-bottom: 18px;
display: flex;
flex-direction: column;
}
.gear-ratio-calculator-container label {
margin-bottom: 8px;
color: #34495e;
font-size: 15px;
font-weight: bold;
}
.gear-ratio-calculator-container input[type="number"] {
width: 100%;
padding: 12px;
border: 1px solid #cccccc;
border-radius: 5px;
font-size: 16px;
box-sizing: border-box;
-moz-appearance: textfield; /* Firefox */
}
.gear-ratio-calculator-container input[type="number"]::-webkit-outer-spin-button,
.gear-ratio-calculator-container input[type="number"]::-webkit-inner-spin-button {
-webkit-appearance: none;
margin: 0;
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.gear-ratio-calculator-container button {
background-color: #007bff;
color: white;
padding: 13px 25px;
border: none;
border-radius: 5px;
cursor: pointer;
font-size: 18px;
font-weight: bold;
width: 100%;
transition: background-color 0.3s ease;
margin-top: 15px;
}
.gear-ratio-calculator-container button:hover {
background-color: #0056b3;
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.gear-ratio-calculator-container .calc-result {
margin-top: 30px;
padding: 20px;
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border-radius: 8px;
background-color: #f9f9f9;
color: #2c3e50;
font-size: 16px;
line-height: 1.6;
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.gear-ratio-calculator-container .calc-result h3 {
color: #007bff;
margin-top: 0;
margin-bottom: 15px;
font-size: 22px;
text-align: center;
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.gear-ratio-calculator-container .calc-result p {
margin-bottom: 10px;
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.gear-ratio-calculator-container .calc-result strong {
color: #0056b3;
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.gear-ratio-calculator-container .calc-result hr {
border: 0;
height: 1px;
background-image: linear-gradient(to right, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.1), rgba(0, 0, 0, 0));
margin: 20px 0;
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/* Article Styling */
.gear-ratio-article h2 {
color: #2c3e50;
font-size: 28px;
margin-top: 40px;
margin-bottom: 20px;
border-bottom: 2px solid #007bff;
padding-bottom: 10px;
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.gear-ratio-article h3 {
color: #007bff;
font-size: 22px;
margin-top: 30px;
margin-bottom: 15px;
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.gear-ratio-article p {
line-height: 1.7;
margin-bottom: 15px;
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function calculateGearRatio() {
var currentGearRatio = parseFloat(document.getElementById("currentGearRatio").value);
var currentTireDiameter = parseFloat(document.getElementById("currentTireDiameter").value);
var desiredTireDiameter = parseFloat(document.getElementById("desiredTireDiameter").value);
var targetSpeed = parseFloat(document.getElementById("targetSpeed").value);
var transmissionRatio = parseFloat(document.getElementById("transmissionRatio").value);
var resultDiv = document.getElementById("gearRatioResult");
resultDiv.innerHTML = ""; // Clear previous results
if (isNaN(currentGearRatio) || isNaN(currentTireDiameter) || isNaN(desiredTireDiameter) || isNaN(targetSpeed) || isNaN(transmissionRatio) ||
currentGearRatio <= 0 || currentTireDiameter <= 0 || desiredTireDiameter <= 0 || targetSpeed <= 0 || transmissionRatio <= 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
// RPM Calculation: RPM = (Speed * Gear Ratio * Transmission Ratio * 336) / Tire Diameter
// 336 is a constant to convert units (miles to inches, hours to minutes, revolutions to rotations).
// Calculate RPM with current setup
var rpmCurrentTires = (targetSpeed * currentGearRatio * transmissionRatio * 336) / currentTireDiameter;
// Calculate RPM with desired new tires (using current gear ratio)
var rpmDesiredTires = (targetSpeed * currentGearRatio * transmissionRatio * 336) / desiredTireDiameter;
// Calculate New Effective Gear Ratio (what the original gear ratio *feels like* with new tires)
// Larger tires reduce effective gearing, so the numerical ratio will be lower.
var newEffectiveGearRatio = currentGearRatio * (currentTireDiameter / desiredTireDiameter);
// Calculate Recommended New Gear Ratio (to restore original RPM at target speed)
// Larger tires require numerically higher gears to maintain RPM.
var recommendedNewGearRatio = currentGearRatio * (desiredTireDiameter / currentTireDiameter);
var output = "Gear Ratio & Tire Size Calculator
Calculation Results:
"; output += "With your current setup (Gear Ratio: " + currentGearRatio.toFixed(2) + ", Tire Diameter: " + currentTireDiameter.toFixed(1) + " inches):"; output += "RPM at " + targetSpeed.toFixed(0) + " MPH (in selected gear): " + rpmCurrentTires.toFixed(0) + " RPM"; output += ""; output += "If you install " + desiredTireDiameter.toFixed(1) + " inch tires with your current " + currentGearRatio.toFixed(2) + " gear ratio:"; output += "Your New Effective Gear Ratio will be approximately: " + newEffectiveGearRatio.toFixed(2) + ""; output += "Your RPM at " + targetSpeed.toFixed(0) + " MPH (in selected gear) will drop to: " + rpmDesiredTires.toFixed(0) + " RPM"; output += "
"; output += "To restore your original RPM (" + rpmCurrentTires.toFixed(0) + " RPM) at " + targetSpeed.toFixed(0) + " MPH with " + desiredTireDiameter.toFixed(1) + " inch tires:"; output += "You would need a Recommended New Gear Ratio of approximately: " + recommendedNewGearRatio.toFixed(2) + ""; resultDiv.innerHTML = output; }
Understanding Gear Ratios and Tire Sizes
When modifying a vehicle, especially for off-road use or performance, changing tire size is a common upgrade. However, larger or smaller tires significantly impact your vehicle's effective gearing, affecting everything from acceleration and fuel economy to speedometer accuracy and engine RPM at cruising speeds. This calculator helps you understand these changes and plan your modifications effectively.
What is a Gear Ratio?
A gear ratio, specifically the differential gear ratio (also known as axle ratio or final drive ratio), describes the relationship between the number of rotations the driveshaft makes and the number of rotations the wheels make. For example, a 3.73 gear ratio means the driveshaft spins 3.73 times for every one rotation of the wheel. A numerically higher gear ratio (e.g., 4.10) provides more torque to the wheels, resulting in quicker acceleration but higher engine RPM at a given speed. A numerically lower gear ratio (e.g., 3.08) offers less torque but lower RPM for better fuel economy at cruising speeds.
What is Tire Diameter?
Tire diameter is simply the overall height of the tire when mounted and inflated. It's usually measured in inches. Larger tires cover more ground with each rotation, while smaller tires cover less.
How Tire Size Affects Your Vehicle's Gearing
Changing your tire diameter directly impacts your vehicle's effective gear ratio.
Larger Tires:
When you install larger diameter tires without changing your differential gears, your vehicle's effective gear ratio becomes numerically lower. This means:
- Reduced Acceleration: The engine has to work harder to turn the larger tires, making the vehicle feel sluggish.
- Lower RPM at Speed: At any given speed, your engine will be turning at a lower RPM. While this might seem good for fuel economy, if the RPM drops too low, the engine might be lugging, which can be inefficient and cause strain.
- Speedometer Error: Your speedometer will read lower than your actual speed.
- Increased Strain: On the transmission and engine, especially when towing or climbing hills.
Smaller Tires:
Conversely, installing smaller diameter tires will result in a numerically higher effective gear ratio. This means:
- Improved Acceleration: The engine can turn the smaller tires more easily, leading to quicker launches.
- Higher RPM at Speed: Your engine will be revving higher at any given speed, potentially reducing fuel economy and increasing engine wear.
- Speedometer Error: Your speedometer will read higher than your actual speed.
Using the Gear Ratio & Tire Size Calculator
Our calculator helps you predict these changes and determine the optimal gear ratio for your desired tire size. Here's how to use it:
- Current Differential Gear Ratio: Enter your vehicle's existing differential gear ratio (e.g., 3.73, 4.10).
- Current Tire Diameter (inches): Input the diameter of the tires currently on your vehicle.
- Desired New Tire Diameter (inches): Enter the diameter of the new tires you are considering.
- Target Speed (MPH): Provide a common cruising speed (e.g., 65 MPH) to compare RPMs.
- Transmission Gear Ratio: This is the ratio of the specific transmission gear you want to analyze (e.g., 1.0 for a 1:1 direct drive gear, 0.7 for a typical overdrive gear). Consult your vehicle's vehicle specifications or owner's manual for these ratios.
Interpreting the Results
The calculator will provide you with several key pieces of information:
- RPM at Target Speed with Current Tires: This shows your baseline engine RPM at your specified cruising speed with your current setup.
- New Effective Gear Ratio: This is what your original differential gear ratio feels like with the new, desired tires. If you go from 30″ to 33″ tires with a 3.73 gear, your effective gear ratio will be numerically lower (e.g., 3.39), indicating a loss of torque.
- RPM at Target Speed with Desired Tires: This shows how much your engine RPM will drop (or increase) at your target speed if you install the new tires without changing your differential gears.
- Recommended New Gear Ratio: This is the differential gear ratio you would need to install to bring your engine RPM back to approximately the same level it was with your original tires at the target speed. This is crucial for restoring performance and maintaining drivability.
By using this calculator, you can make informed decisions about tire and gear changes, ensuring your vehicle performs optimally for your specific needs, whether it's for daily driving, towing, or off-road adventures.