Speedometer Error Calculator

Taller tire: you are going faster than the needle. Shorter tire: you are going slower.

Try stock 265/70R17 vs 285/70R17 — a common mild lift in diameter.

A diameter change moves ride height by half
+1"+0.5"originaltaller
  • The tire rests on the ground, so half of any diameter change goes downward and only the other half lifts the vehicle.
  • A diameter one inch larger raises the vehicle half an inch, and takes half an inch of fender clearance away.
  • That same change shifts the speedometer and revolutions per mile, because the tire covers more ground per turn.
  • Width is a separate problem: a wider tire can rub even when the diameter is perfect.

What it does

Uses the ratio of overall diameters to show indicated vs actual speed at everyday US speeds, plus how the odometer counts miles.

How to use it

Enter the stock size (what the speedometer was calibrated for) and the size on the truck now.

How to read the result

If the new tire is 3% taller, 65 mph on the needle is about 67 mph true, and 100 indicated miles are about 103 miles traveled. Police, warranties and service intervals care about real distance. GPS is the easy check.

What this does not calculate

Assumes the stock size was calibrated correctly and ignores wear, load squat and aftermarket speedometer calibrators. Geometric diameter, not rolling radius.

Does not reprogram your ECU or tell you which speedometer calibrator to buy.

Questions

Does this also affect the odometer?
Yes, by the same ratio. A taller tire under-reports miles. Oil-change reminders will come late in real distance.
Will ABS or traction control care?
They use wheel-speed sensors. A large diameter change can confuse them. That is one reason to stay near 3%.

Related tools

Estimate — not a fitment approval.

Speedometer Error Calculator