Use this calculator to determine how changing your tire size will affect your vehicle's speedometer reading. An inaccurate speedometer can lead to speeding tickets or incorrect distance tracking.
Original Tire Size
New Tire Size
Understanding Your Tire Speedometer Calculator
When you change the size of your vehicle's tires, you're also changing the overall diameter of the wheel. Since your car's speedometer measures speed based on the rotation of the wheels and assumes a specific tire diameter, altering this diameter will cause your speedometer to become inaccurate.
Why Tire Size Matters for Speedometer Accuracy
Your vehicle's computer is calibrated from the factory for a specific tire size. Each rotation of the wheel is translated into a certain distance traveled. If you install tires with a larger overall diameter, each rotation will cover more ground than the car's computer expects. This means that for a given number of rotations, your car will actually be traveling faster than what your speedometer indicates (it will under-report your speed).
Conversely, if you install tires with a smaller overall diameter, each rotation will cover less ground. In this case, your car will be traveling slower than what your speedometer indicates (it will over-report your speed).
How to Read Tire Sizes
Tire sizes are typically displayed in a format like P205/55R16:
205: This is the tire's width in millimeters (mm).
55: This is the aspect ratio, representing the sidewall height as a percentage of the tire's width. So, the sidewall height is 55% of 205mm.
R: Indicates a radial construction tire.
16: This is the diameter of the rim (wheel) in inches.
The calculator uses these three key numbers (width, aspect ratio, and rim diameter) to determine the overall tire diameter.
The Calculation Explained
The core of the calculation involves determining the overall diameter of both your original and new tires. The formula for overall tire diameter is:
Once both diameters are known, the calculator determines the ratio of the new tire's diameter to the original tire's diameter. This ratio is then applied to your speedometer reading to find your actual speed and the percentage of speedometer error.
Implications of an Inaccurate Speedometer
Speeding Tickets: If your speedometer reads lower than your actual speed, you could unknowingly be exceeding speed limits.
Safety: Incorrect speed readings can affect your judgment of safe following distances and braking times.
Odometer Inaccuracy: Your odometer, which tracks mileage, will also be affected, leading to incorrect records for maintenance, warranty, and resale value.
Fuel Economy: While not directly affected by the speedometer, changes in tire size can sometimes impact fuel efficiency dueating to changes in rolling resistance or gearing.
How to Use This Calculator
Enter Original Tire Size: Input the width (mm), aspect ratio (%), and rim diameter (inches) of the tires that came with your vehicle or the tires you had before the change.
Enter New Tire Size: Input the corresponding details for the new tires you have installed or are considering installing.
Enter Speedometer Reading: Input a typical speed you might be traveling (e.g., 60 mph) to see the actual speed at that reading.
Click "Calculate": The calculator will then display your original and new tire diameters, the percentage difference, and most importantly, your actual speed when your speedometer shows the entered reading.
Always verify your speedometer accuracy with a GPS device or a professional calibration after changing tire sizes, especially if the change is significant.