Speedometer Calculator for Tire Size Changes
See your actual speed after switching tire size.
Set up for: speedometer after a size change · Different situation?
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). The placard shows the size the vehicle came with; the sidewall shows what is fitted now. If they differ, use the placard as your starting point. 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.