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The size on the door placard, not the one on the car.
Your own units throughout — the error is a percentage either way.
To see what the odometer would have recorded over it.
0.38%
The larger fitted tyre covers more ground per turn, so the dial now reads lower than the true speed.
This is the error the size change adds, and it is measured against a perfect instrument. A real speedometer is not perfect and was never meant to be: in most jurisdictions the rule is one-sided, requiring that the indicated speed is never lower than the true speed, so a car on its original tyres already reads a little high. How much margin is permitted is set by regulation where you drive, and no figure for it is quoted here because it varies and would need a citation. Your dial’s own error stacks on top of the percentage above.
Fit a bigger rolling diameter and the two errors work against each other, eating the built-in margin and eventually pushing the dial into under-reading — the direction no regulator allows. Fit a smaller one and they compound, so the car reads even further above the truth than it did. That is the practical reason the direction of the change matters as much as its size, and the reason a change is worth measuring against a satellite speed readout on a road you know rather than trusting either instrument alone.
The odometer and the trip meter run off the same revolution count, so both carry the same percentage in the same direction. Over a service interval or a lease mileage allowance that adds up quietly, and the car’s recorded mileage is the figure that follows it around. Whether a size belongs on the vehicle at all is the manufacturer’s call and a tyre professional’s, never this page’s.
A speedometer has no idea how fast the car is moving. It knows how quickly something is rotating — a wheel, a driveshaft, an ABS reluctor ring — and multiplies that rate by a constant chosen for the rolling diameter the vehicle was built around. The constant is fixed. The moment a different rolling diameter is bolted on, each revolution covers a different amount of road than the constant assumes, and the needle is answering a question about the old tyres.
Fit something taller and the car travels further per turn than the instrument credits it for, so the dial now reads below the true speed. Fit something shorter and the dial reads above it. The size of the effect is the ratio between the two rolling diameters and nothing else.
A vehicle calibrated on 215/65R16 rolls 685.9 mm: a 139.75 mm sidewall either side of a 406.4 mm rim. Move to 235/65R17 and the rolling diameter becomes 737.3 mm. Divide the second by the first and the ratio is 1.0749, so the fitted size introduces an error of 7.49%.
Read that in both directions, because both are asked. With the needle on 70, the car is really doing 75.2. To actually be doing 70, the needle has to sit at 65.1. And over 100 units of road genuinely covered, the odometer — which counts the same revolutions through the same constant — logs only 93.0 of them.
None of the above is measured against your dial. It is measured against a perfect instrument, and no production speedometer is one. In most jurisdictions the legal requirement is deliberately one-sided: the indicated speed may not be lower than the true speed, though it is allowed to sit above it. A car on its original tyres therefore reads slightly fast on purpose.
How much margin is permitted is a regulatory figure that differs between countries and is periodically revised, so no number for it appears anywhere on this page. Anyone quoting you one without naming the regulation it comes from is guessing. What matters here is the direction: a taller tyre spends that built-in margin and can push an instrument into under-reading, which is the one direction the rules do not permit.
The percentage above is the change your tyres introduced, stacked on top of whatever error the instrument already had. The two are independent, and only one of them is calculable. The way to find the total is to compare the dial against an independent ground-speed measurement on a road you know, at a steady speed, more than once.
Nothing on this page approves a size. Whether a rolling diameter belongs on a vehicle is set by that vehicle manufacturer and confirmed by a tyre specialist, and a change large enough to matter to the speedometer is usually large enough to matter to the arches, the ABS calibration and the load the tyre is carrying as well.
They make it geared longer, which is not the same thing. Each engine revolution now covers more ground, so acceleration softens and cruising revs drop. Top speed may rise or fall depending on where the engine makes its power. What definitely changes is that the dial stops telling the truth about any of it.
Almost certainly not, and it may work against you. Enforcement measures the speed of the vehicle directly rather than reading your instrument, and a dial that under-reads because of a fitment choice is generally treated as the driver’s responsibility. Treat the figure as a reason to correct the car, not as a defence.
Yes. The odometer and the trip meter are driven by the same revolution count through the same constant, so both carry the identical percentage in the identical direction. On a lease mileage allowance or a service interval that accumulates quietly, and the recorded mileage is the figure that follows the vehicle around afterwards.
It is a genuinely independent measurement, which is why it is worth comparing against, but it is not exact either. It derives speed from position over time, so it lags under acceleration, wanders where reception is poor and reports speed over the ground rather than along a gradient. Use it as a second opinion.