Speedometer Error Calculator

Getting a number you can act on

1. 01
   Find the size the vehicle was calibrated for

   It is on the tire and loading placard, usually in the driver's door jamb. This is the original equipment size, which is not always what is fitted now — and the calibration follows the placard, not the rubber.

2. 02
   Enter the size actually on the vehicle

   Read it off the sidewall. If someone changed sizes before you bought the vehicle, this is where the discrepancy shows up.

3. 03
   Read the error percentage first

   It is a fixed proportion, not a fixed number of miles per hour. That means the absolute error grows the faster you drive — an issue at highway speed even when it looks small around town.

4. 04
   Check the direction, not just the size of the error

   Taller tires make the speedometer read low, so you are going faster than it shows. Shorter tires make it read high. The direction is what decides whether the error works for you or against you.

5. 05
   Decide whether to recalibrate

   Many vehicles allow the tire size to be reset by a dealer or an aftermarket tool. That corrects the speedometer, the odometer and anything else deriving speed from wheel rotation in one operation.

What the calculator is actually doing

Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.

Why the error is a percentage and not a fixed amount

The most useful thing to understand here is that speedometer error scales. It is not a constant offset of two or three miles per hour that you can memorise and mentally subtract.

The instrument counts wheel rotations and multiplies by an assumed distance per rotation. If the real tire covers 4% more ground per rotation than assumed, then every rotation contributes 4% more distance than the instrument credits — at 20 mph and at 80 mph alike.

In practice that means a 4% error is 0.8 mph at an indicated 20, and 3.2 mph at an indicated 80. The percentage never changes; the number of miles per hour it represents does. That is why an error that feels trivial in town can matter on a motorway, and why the table above lists several speeds rather than one.

Which direction the error runs

Taller tires cover more ground per revolution than the calibration expects, so the vehicle is going faster than the dial shows. This is the direction most people encounter, because most size changes are upward.

Shorter tires do the reverse. The wheel turns more times to cover the same ground, the instrument counts more rotations, and the dial reads higher than your actual speed.

The direction matters more than the magnitude for some purposes. A speedometer reading low is the one that costs you a ticket, because you are travelling faster than you believe. A speedometer reading high is the one that costs you resale value, because the odometer is adding distance the vehicle never covered.

Worth knowing: most manufacturers calibrate speedometers to read slightly high from the factory — typically a couple of percent — precisely so that the instrument never understates your speed. Fitting slightly taller tires can therefore make an indicated speed more accurate rather than less, up to a point.

Regulation is why your speedometer already reads high

Before blaming a tire change for an inaccurate speedometer, it is worth knowing that the instrument was never designed to be accurate in the first place — it was designed to be safe.

UN Regulation No. 39, which governs speedometer equipment across Europe and much of the world, states the requirement plainly: the indicated speed must never be lower than the true speed. There is no allowance for reading low, at all. The permitted error runs one way only.

The regulation caps that positive error too. Indicated speed must not exceed the true speed by more than 10 percent plus 4 km/h. So at a true 100 km/h a compliant speedometer may legitimately display anything from 100 up to about 114 km/h, and manufacturers typically build in a couple of percent of deliberate over-reading to stay clear of the boundary.

That has a consequence people rarely anticipate. Because the factory reading is already optimistic, fitting slightly taller tires — which pushes the reading downward — can move an indicated speed closer to the truth rather than further from it. Up to a point, a small increase corrects an error that was built in on purpose. Past that point it starts eating into a margin that exists for a reason, and the regulation's floor is where it stops being harmless.

What else derives from the same wheel-speed signal

• The odometer, which drifts by the same percentage over distance rather than time
• Cruise control, which will hold a true speed different from the one you set
• Trip computer fuel economy, which divides recorded distance by fuel used
• Anti-lock braking and stability control, which compare wheel speeds against expected values
• Adaptive cruise and lane-keeping systems, which use vehicle speed as an input
• Warranty and lease mileage, where under-recording quietly accumulates in your favour and over-recording against you

When recalibration is worth doing

If the vehicle supports it, recalibration is almost always the right answer for anything beyond about 3%. It is a software change rather than a mechanical one on most modern vehicles, and it fixes every downstream system at once rather than leaving you doing mental arithmetic.

The exceptions are older vehicles, where correction may mean changing a physical drive gear in the transmission, and vehicles where the calibration is locked. In those cases the practical approach is to know the number and account for it — particularly for the odometer, where the drift compounds silently over years.

It is also worth recalibrating before selling. An odometer reading that is several percent adrift is a genuine discrepancy in the vehicle's recorded history, and correcting it while you still own the vehicle is considerably simpler than explaining it afterwards.