How to Calculate Speedometer Error: Formula, Worked Example & Tables

The speedometer error formula is two short calculations, and this page works through both with real tyre sizes so the result can be checked by hand. Error % = (new diameter − old diameter) ÷ old diameter × 100 gives the percentage, and actual speed = indicated speed × (new diameter ÷ old diameter) gives the true speed at any indicated speed. The worked example uses 205/55R16 (631.9 mm) replaced by 215/55R16 (642.9 mm), a +1.74% change, and the tables show the indicated-versus-actual speeds at 30, 50 and 70 mph. All diameters are nominal new-tyre figures computed from the size codes; real tyres vary slightly by brand, model, wear and inflation pressure. That is the point this page works through for how to calculate speedometer error.

The formula is worth knowing because it answers every speedometer question from the same two numbers: the old diameter and the new diameter. Change either one and the answer follows, whether the question is about a single size change, a correction factor for a programmer, or the odometer error over a year of driving. The sections below show the arithmetic step by step, the tables for the worked example, the common causes of a wrong reading, how to check the dial against GPS, and when a recalibration is needed — with the disclaimer below applying throughout. That is the point this page works through for how to calculate speedometer error.

This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.

how to calculate speedometer error: How the speedometer error is calculated

Work it out in two steps. Step one, the percentage: error % = (new diameter − old diameter) ÷ old diameter × 100, using the nominal new-tyre diameters computed from the size codes. Step two, the actual speed at any indicated speed: actual speed = indicated speed × (new diameter ÷ old diameter). A tyre that is 2% larger in diameter gives an actual speed 2% above the indicated speed — 71.4 mph actual at an indicated 70 mph — and a tyre 2% smaller gives 68.6 mph actual at the same indicated speed. The odometer follows the same percentage, so the mileage it records is also out by that amount. All the tables below are built from those two formulas with the diameters stated. That is the point this page works through for how to calculate speedometer error.

how to calculate speedometer error: What the speedometer actually measures

Taking how to calculate speedometer error as the case in point: Speedometer error is a percentage, not a fixed number of miles per hour, and that is the key to understanding it. The dial shows the speed the car would be doing if the original tyres were fitted, and the actual speed is the indicated speed multiplied by the ratio of the new diameter to the old diameter. A small diameter change gives a small error — a 1% change is 0.7 mph at an indicated 70 mph — while a large change gives an error that matters for speed limits and for the odometer. Because the error is proportional, the same table works at any speed once the percentage is known, and the worked examples in this article show how to find that percentage from the two tyre sizes.

Worked example: 205/55R16 to 215/55R16 for how to calculate speedometer error

On this page, how to calculate speedometer error is the case in point: 205/55R16 measures 631.9 mm in overall diameter and 215/55R16 measures 642.9 mm. The error % is (642.9 − 631.9) ÷ 631.9 × 100 = +1.74%. At an indicated 30 mph the actual speed is 30 × (642.9 ÷ 631.9) = 30.5 mph; at 50 mph it is 50.9 mph; and at 70 mph it is 71.2 mph. The odometer is out by the same 1.74%: it records 98.29 miles for every 100 miles actually driven. The diameter change of 1.74% is inside the ±3% guideline commonly used for keeping speedometer error small, which is why this pair is a useful example — a larger change, such as 235/65R17 (737.3 mm) to 265/70R17 (802.8 mm), is +8.89% and gives 76.2 mph actual at an indicated 70 mph.

Indicated vs actual speed: the worked example for how to calculate speedometer error

Error: +1.74%. Actual speed = indicated speed × (642.9 ÷ 631.9). That is the point this page works through for how to calculate speedometer error.

Indicated speed Actual speed Difference
30 mph 30.5 mph +0.5 mph
50 mph 50.9 mph +0.9 mph
70 mph 71.2 mph +1.2 mph

The table is read from the indicated speed: find the dial reading and read across for the actual speed. The difference column is the amount by which the car is travelling faster than the dial shows, and it grows with speed because the error is a percentage. A 1.74% error is 0.5 mph at 30 mph and 1.2 mph at 70 mph — small in both cases, but the direction is the one that matters, because the car is going faster than the driver believes. Check the door-jamb placard for the specified size and confirm fitment with a tyre professional before fitting a different tyre size. That is the point this page works through for how to calculate speedometer error.

Common causes of a wrong speedometer for how to calculate speedometer error

Taking how to calculate speedometer error as the case in point: Several things can put a speedometer out, and they are worth separating because the fixes differ. The first is tyre diameter: any change from the size the vehicle was calibrated for produces a fixed percentage error, and it is the cause this article is about. The second is wear and pressure: a worn tyre is smaller than a new one and an under-inflated tyre has a smaller effective rolling radius, so both make the dial read slightly high. The third is a previous recalibration — a programmer setting or a speedometer gear fitted for an earlier tyre size that no longer matches what is on the car. The fourth is a fault: a failing wheel-speed sensor, a damaged reluctor ring or, on older cars, a worn cable, which usually shows as a needle that jumps, sticks or reads inconsistently rather than a steady percentage error. A steady percentage error points at calibration; an erratic needle points at a fault.

How to fix a speedometer error for how to calculate speedometer error

For how to calculate speedometer error, the point is this: There are four common ways to correct a speedometer after a tyre change, and the right one depends on the vehicle. A handheld programmer or diagnostic tool can change the programmed tyre size on many modern vehicles, which corrects both the speedometer and the odometer at once. A speedometer drive gear — a small gear in the gearbox — does the same job mechanically on older vehicles, and the replacement gear is chosen from the diameter ratio. An inline calibrator, wired into the speed sensor signal, can scale the signal on vehicles where neither of the above is possible. And a dealer recalibration uses manufacturer software to change the setting where it is locked to franchised tools. Before spending money on any of them, measure the error against GPS at a steady speed so the correction needed is known, and confirm the method suits the vehicle — the wrong tool for the vehicle wastes the money and leaves the dial wrong.

Common mistakes with speedometer error for how to calculate speedometer error

Taking how to calculate speedometer error as the case in point: Mistakes usually start with the wrong reference. The first is comparing the dial with another vehicle's dial — both can be out by the same amount. GPS speed, measured at a steady speed on a straight road, is the practical reference, though it has its own limits at low speed and on hills. The second mistake is treating the error as a fixed mph figure: a 3% error is 0.9 mph at 30 mph and 2.1 mph at 70 mph, so a single-speed check does not describe the whole picture. The third is forgetting that most speedometers already read slightly high from the factory, so the error after a tyre change is measured from that baseline, not from zero. And the fourth is the dangerous one: fitting larger tyres makes the dial read low, so the car is going faster than the driver believes, and driving to the indicated limit in that condition means exceeding it.

Related pages for how to calculate speedometer error

Frequently asked questions

How do I calculate speedometer error for how to calculate speedometer error?

Taking how to calculate speedometer error as the case in point: Use error % = (new diameter − old diameter) ÷ old diameter × 100, then actual speed = indicated speed × (new diameter ÷ old diameter). The worked example above uses 205/55R16 (631.9 mm) and 215/55R16 (642.9 mm), a +1.74% change, and the speedometer error calculator runs the same formula with your own sizes.

How much can a speedometer be out?

Taking how to calculate speedometer error as the case in point: It depends on the tyre change. A 1% diameter change is 0.7 mph at an indicated 70 mph; a 3% change is 2.1 mph; an 8.89% change, such as 235/65R17 to 265/70R17, is 6.2 mph at an indicated 70 mph. In the UK the law requires that a speedometer never under-reads and may over-read by up to 10% plus 4 km/h.

Why is my speedometer wrong after new tyres?

For how to calculate speedometer error, the point is this: The usual reason is a different diameter from the size the vehicle was calibrated for: a larger tyre makes the dial read low and a smaller tyre makes it read high, by the diameter percentage. Other causes are wear, pressure, a previous recalibration for a different size, or a sensor fault. Measure the error against GPS at a steady speed to find out which it is.

How do I check my speedometer against GPS?

Drive at a steady speed on a straight, level road and compare the dial with a GPS speed reading. GPS is least reliable at low speed, while accelerating or braking, and on hills or in tunnels, so take the reading at a steady cruise. A difference that is the same percentage at several speeds is a calibration error; a needle that jumps or sticks is a fault. That is the point this page works through for how to calculate speedometer error.

Does tyre wear affect the speedometer?

Yes, slightly. A worn tyre has a smaller diameter than a new one, so the speedometer reads slightly high as the tread wears — typically well under 1% over the life of the tyre. Under-inflation has the same direction of effect by reducing the effective rolling radius. Both are small compared with a size change, but they are real. That is the point this page works through for how to calculate speedometer error.