Original tire size
mm
%
in
New tire size
mm
%
in
Diameter change
–
Speedometer at true 100
–
Revs per km change
–
How much size difference is safe?
Most manufacturers and tire shops treat up to about a 3% overall-diameter difference as safe for speedometer accuracy and clearance; this calculator shows your exact percentage.
Why does my speedometer read wrong after a tire change?
The speedometer is calibrated for the original tire's revolutions per kilometre. A larger tire covers more ground per revolution, so the car is actually going faster than the speedometer shows — and vice versa for smaller tires.
How it works
Enter your current tire size and the new size you’re considering. The calculator compares overall diameter, so you can catch a mismatch before it affects your speedometer or ground clearance.