Tire RulerHow the speedometer error works
Your speedometer counts wheel (or transmission) revolutions and assumes the original tire’s circumference. A taller tire travels farther per revolution, so the real speed is higher than the reading:
real speed = indicated speed × new diameter ÷ original diameter
Example: 265/70R17 → 285/70R17
- Diameters: 31.61 in and 32.71 in.
- 60 × 32.71 ÷ 31.61 = 62.1 mph
The error is the same percentage at every speed, so it grows in mph as you go faster. Many places don’t accept “my tires are bigger” as an excuse for a speeding ticket, so it’s worth recalibrating after a big change.
Frequently asked questions
Do bigger tires make my speedometer read slower?
Yes. A taller tire covers more ground per turn, so you travel faster than the speedometer shows. With 3.5% taller tires, an indicated 60 mph is about 62 mph.
Do bigger tires affect the odometer?
Yes, by the same percentage. With 3.5% taller tires, the odometer records about 96.6 miles for every 100 miles you actually drive.
How do I fix my speedometer after new tires?
On most modern vehicles the tire size is a setting: a dealer, a tuner or a plug-in programmer can change it. Older vehicles may need a different speedometer drive gear.
Is a GPS speed more accurate than my speedometer?
On a straight, level road with good signal, GPS speed is a good check. Compare it with this calculator’s result at a steady speed.