Speedometer Error Calculator
One percentage, expanded into the table you actually need — because the error you care about is the one at the speed you drive.
Reading error
+3.49%
The speedometer under-reads: the vehicle is going faster than the needle says.
Indicated against actual
| Indicated | Actual | Difference |
|---|---|---|
| 20 mph | 20.7 mph | +0.7 |
| 30 mph | 31.0 mph | +1.0 |
| 40 mph | 41.4 mph | +1.4 |
| 50 mph | 51.7 mph | +1.7 |
| 55 mph | 56.9 mph | +1.9 |
| 60 mph | 62.1 mph | +2.1 |
| 65 mph | 67.3 mph | +2.3 |
| 70 mph | 72.4 mph | +2.4 |
| 75 mph | 77.6 mph | +2.6 |
| 80 mph | 82.8 mph | +2.8 |
Odometer: an indicated 100 miles is 103.5 actual miles.
How to read the result
The single percentage at the top is the reading error, and its sign is doing real work. A positive figure means the fitted tire is the taller of the two, so every wheel revolution covers more ground than the instrument was calibrated to assume — and the result line above spells out which way that sends the reading. A negative figure is the shorter-tire case, and everything below inverts.
The table expands that percentage across the range, because a percentage is hard to feel. Two per cent is a rounding error at 30 mph and is most of a mile an hour at 75.
The last line is the odometer, which is the same error accumulating. It is the one people forget, and it is the one that costs money: a warranty interval, a lease mileage allowance and a service schedule are all counted by an instrument that is now wrong by a fixed proportion.
The arithmetic, in one line
actual speed = indicated speed x fitted diameter / original diameter
That is the entire calculation. A speedometer is driven by wheel rotation and is calibrated to one rolling circumference; change the circumference and the reading scales by exactly that ratio. Because circumference is pi times diameter, the pi cancels and only the diameter ratio remains.
It makes no difference whether the signal comes off a cable or an anti-lock brake sensor. Either way the instrument counts a fixed number of events per turn of the wheel and multiplies, so it has no way to know the wheel got bigger. A 1974 truck and a 2024 one are wrong by the same ratio.
A worked example
Load 265/70R17 as the original and 285/70R17 as the fitted size. The diameters are 31.61 in
and 32.71 in, so the error is +3.49%. An indicated 70 mph is an actual 72.4 mph, and an indicated
100 miles is 103.5 miles on the road.
Step up to 35x12.50R17 from the same 265 and the error becomes +10.74%. Indicated 70 is an actual
77.5 mph. That is the difference between a speed limit and a citation, and it is why the error is
worth knowing before the tires go on rather than after.
What this does not tell you
Whether the reading can be corrected, and how. Some vehicles accept a recalibration through the factory diagnostic tool, some need an aftermarket module, and some cannot be changed at all without a different instrument cluster. That is a question about your specific vehicle, and it is not derivable from two tire sizes.
It also says nothing about whether the fitted size belongs on the vehicle. That depends on clearance, load rating and local rules — none of which are geometry.