Speedometer Calculator for Tire Size Changes
See your actual speed after switching tire size.
Your current size
e.g. 265/70R17, 2657017 or 33x12.50R20
Where do I find my size?
It's on the tire sidewall, e.g. P265/70R17 115T, and on the tire placard sticker on the driver's door jamb (also in the owner's manual). If your current tires aren't the original size, the placard shows what the vehicle came with. Digits are enough here: 2657017 or 33125020.
Size you're considering
How it works
The speedometer counts wheel turns and assumes the original tire's circumference. A tire with a +3.0% larger diameter covers 3% more ground per turn, so actual speed = indicated × new diameter ÷ old diameter. This is an estimate from the size change only: factory speedometers have their own tolerance, real tires differ from nominal, and recalibration options differ by vehicle.
FAQ
How do I work out speedometer error after changing tire size?
Actual speed = speedometer reading × new diameter ÷ old diameter. A tire 3% taller makes the speedometer read 3% low: at an indicated 60 mph you are doing about 61.8 mph.
Do bigger tires make the speedometer read slower?
Yes. A taller tire travels farther per turn, so for the same wheel speed you're going faster than the speedometer shows. Smaller tires make it read high.