Speedometer Correction Calculator

When you change your tire size, your speedometer reading can become inaccurate. This calculator helps you determine the true speed of your vehicle with new tires.

Original Tire Size

Section width (e.g., 225)

Aspect ratio (e.g., 60)

Rim diameter (e.g., 17)

Example format: 225/60R17

New Tire Size

Section width (e.g., 245)

Aspect ratio (e.g., 50)

Rim diameter (e.g., 18)

Example format: 245/50R18

Original Tire Diameter (mm) Enter your original tire's overall diameter in millimeters.

New Tire Diameter (mm) Enter your new tire's overall diameter in millimeters.

Speedometer Reading (mph) Enter the speed shown on your speedometer to calculate your actual speed.

About Speedometer Accuracy

When you change your tire size, your vehicle's speedometer reading can become inaccurate because it's calibrated based on the original tire's diameter. Here's why:

Your speedometer measures the rotational speed of your transmission or wheels
It converts this to vehicle speed based on the assumption that your tires have a specific diameter
Larger diameter tires travel farther in one rotation than smaller diameter tires
Therefore, changing tire size affects the relationship between wheel rotation and actual vehicle speed

How Tire Size Affects Speedometer Reading

Larger Tires: Your actual speed will be faster than what your speedometer shows
Smaller Tires: Your actual speed will be slower than what your speedometer shows

Important Safety Considerations

Speedometer inaccuracy can lead to unintentionally speeding or driving too slowly, which can be dangerous and may result in traffic violations.

If you've changed your tire size significantly:

Consider getting your speedometer recalibrated professionally
Use a GPS device to verify your actual speed
Adjust your driving accordingly based on the calculations from this tool

Remember: In many jurisdictions, having an inaccurate speedometer is not a legal defense for speeding tickets.

Speedometer Correction Calculator