Diameter difference: The total change in tire height between your original and new tire. A positive value means the new tire is taller; negative means shorter.
Speedometer error: The percentage your speedometer will read incorrectly. A positive percentage means your speedo reads slower than actual speed (larger tire), so you are going faster than displayed.
Revs per mile: How many times each tire rotates over one mile. Larger tires make fewer revolutions, which also affects odometer accuracy.
Clearance change: How much higher or lower the fender sits relative to the axle center. Half the diameter difference translates directly to clearance change on each side.
How This Calculator Works
You enter the width (mm), aspect ratio (%), and rim diameter (in) for two tires. The tool converts each sidewall height from millimeters to inches, adds twice the sidewall to the rim diameter to get overall diameter, then compares the two. Speedometer error is the percentage difference in circumference. It assumes both tires are inflated to spec and have full tread depth.
Quick Questions
How much speedometer error is acceptable?
Most experts recommend staying within 3% speedometer error when changing tire sizes. Beyond that, you risk speeding tickets and inaccurate odometer readings that can affect warranty and resale value.
What does the tire size "225/65R17" mean?
The first number (225) is tread width in millimeters, the second (65) is the sidewall height as a percentage of width, and the last (17) is the rim diameter in inches. "R" means radial construction.
Will a bigger tire affect my fuel economy?
Generally yes. Larger, heavier tires increase rolling resistance and rotational mass, which can reduce fuel economy by 1–3%. The effect depends on the size difference and driving conditions.
Do I need to recalibrate my speedometer after changing tire size?
If the diameter difference exceeds about 3%, recalibration is recommended. Many modern vehicles allow a dealer or mechanic to reprogram the speedometer through the ECU. Some aftermarket GPS speedometers also work.