This is your current tire's actual speed at 60 mph indicated on the speedometer.
This is the desired diameter of your new tires.
Understanding Tire Size and Speedometer Accuracy
When you change the overall diameter of your tires, it directly impacts the accuracy of your vehicle's speedometer and odometer. The speedometer is calibrated by the manufacturer based on the original tire size. If you install tires that are larger or smaller than the stock size, your speedometer will either read higher or lower than your actual speed. This calculator helps you understand how a change in tire diameter will affect your indicated speed.
The Math Behind the Calculation
The fundamental principle is that the number of rotations a tire makes over a given distance is inversely proportional to its circumference (and thus its diameter). A larger tire covers more ground with each rotation than a smaller tire.
The relationship between tire diameter and indicated speed can be calculated using a simple ratio:
In this calculator, we use your Current Tire Diameter and the Target Tire Diameter. We then use your provided Speedometer Reading (at 60 mph) to determine how much your speedometer will be off with the new tires.
How to Use This Calculator:
Current Tire Diameter: Find the overall diameter of your vehicle's original equipment (OE) tires. This is often found in your owner's manual or online specifications for your vehicle model. It's usually measured in inches (e.g., 26.0 inches).
Speedometer Reading (at 60 mph): This is the speed indicated on your speedometer when your vehicle is *actually* traveling at 60 mph with your *current* tires installed. This is crucial for accurate calibration. If you don't know this value, you can estimate it using online calculators that take your current tire size and gear ratio, or by using a GPS device to measure your actual speed. For simplicity, many assume this is exactly 60 mph, but this calculator allows for more precise calibration.
Target Tire Diameter: Determine the overall diameter of the new tires you are considering. This information is typically available from the tire manufacturer's specifications or can be calculated from the tire's P-metric or Euro-metric size (e.g., 275/65R18).
Interpreting the Results
The calculator will output your Actual Speed when your speedometer reads 60 mph with the new, target tire size installed.
If the calculated Actual Speed is less than 60 mph, your speedometer will read higher than your actual speed.
If the calculated Actual Speed is greater than 60 mph, your speedometer will read lower than your actual speed.
This information is vital for maintaining legal speed limits, accurate mileage tracking, and ensuring proper operation of vehicle systems that rely on accurate speed data (like cruise control and ABS).
function calculateSpeedometerAdjustment() {
var currentTireDiameter = parseFloat(document.getElementById("currentTireDiameter").value);
var speedometerReading = parseFloat(document.getElementById("speedometerReading").value);
var targetTireDiameter = parseFloat(document.getElementById("targetTireDiameter").value);
var resultDiv = document.getElementById("result");
// Clear previous result
resultDiv.innerHTML = "";
// Input validation
if (isNaN(currentTireDiameter) || currentTireDiameter <= 0) {
resultDiv.innerHTML = "Please enter a valid current tire diameter.";
return;
}
if (isNaN(speedometerReading) || speedometerReading <= 0) {
resultDiv.innerHTML = "Please enter a valid speedometer reading.";
return;
}
if (isNaN(targetTireDiameter) || targetTireDiameter <= 0) {
resultDiv.innerHTML = "Please enter a valid target tire diameter.";
return;
}
// Calculation
var actualSpeed = speedometerReading * (currentTireDiameter / targetTireDiameter);
// Display result
resultDiv.innerHTML = "" + actualSpeed.toFixed(1) + " mph(Your actual speed when speedometer reads 60 mph)";
}