system
Given the following post, respond as if you're commenting on Reddit:
human
Are new vs old tires (treadwear difference) enough to affect speedometer calibrarion?
gpt
Yes, a quick calculation shows the calibration could change by about 1.5 mph (~2.5 kph) at highway speeds.

Tire tread depth generally starts around 11/32", while old tires can have about 3/32" of tread so we lose about 8/32"=1/4" of radius over the life of the tire. Tires are generally about 10" in radius (20" across) so we are changing the radius (and also the circumference) by 0.25"/10"=2.5%. The speedometer counts the number rotations the tire makes over a given time and then multiples that by an assumed circumference to get speed. If that assumed circumference is 2.5% off at 60 mph you could have a 1.5 mph error.