Speedometer Error Chart: Error Percentages & Speed Tables

For speedometer error chart, the point is this: A speedometer error chart shows the actual speed for each indicated speed at each error percentage, so the effect of a tyre change can be read off without doing the arithmetic each time. This page is that chart: the formula behind it, error % = (new diameter − old diameter) ÷ old diameter × 100, and the tables for error percentages from −3% to +3% at 30, 50 and 70 mph, plus a worked example using 205/55R16 (631.9 mm) and 215/55R16 (642.9 mm), a +1.74% change. Find the error percentage from the two tyre sizes, then read the actual speed from the chart.

On this page, speedometer error chart is the case in point: The chart also shows why the error matters more at higher speeds: a 3% error is 0.9 mph at an indicated 30 mph but 2.1 mph at an indicated 70 mph, because the error is a percentage of the speed. The same percentage applies to the odometer, so the mileage error grows with the distance driven in the same way. The sections below explain how to find the error percentage from any two sizes, how to check the dial against GPS, what the law requires, and when a recalibration is worth paying for.

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 the speedometer error is calculated for speedometer error chart

Two formulas cover everything on this page. Error % = (new diameter − old diameter) ÷ old diameter × 100, and actual speed = indicated speed × (new diameter ÷ old diameter). The diameters are the nominal new-tyre figures computed from the size codes — for example 205/55R16 measures 631.9 mm and 225/45R17 measures 634.3 mm — and real tyres vary slightly by brand, model, wear and inflation pressure, so the error figures are accurate to within a small margin rather than exact to the tenth of a percent. A positive error % means the new tyre is larger and the car travels faster than the dial shows; a negative error % means the new tyre is smaller and the car travels slower than the dial shows. The odometer error is the same percentage in the same direction. That is the point this page works through for speedometer error chart.

speedometer error chart: What the speedometer actually measures

On this page, speedometer error chart is 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 speedometer error chart

For speedometer error chart, the point is this: 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 speedometer error chart

Taking speedometer error chart as the case in point: Error: +1.74%. Actual speed = indicated speed × (642.9 ÷ 631.9).

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

For speedometer error chart, the point is this: 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.

speedometer error chart: Common causes of a wrong speedometer

Taking speedometer error chart 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 speedometer error chart

On this page, speedometer error chart is the case in point: 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.

speedometer error chart: Common mistakes with speedometer error

Taking speedometer error chart 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 speedometer error chart

Frequently asked questions

How do I calculate speedometer error for speedometer error chart?

Taking speedometer error chart 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?

For speedometer error chart, the point is this: 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?

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. That is the point this page works through for speedometer error chart.

How do I check my speedometer against GPS?

On this page, speedometer error chart is the case in point: 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.

Does tyre wear affect the speedometer?

On this page, speedometer error chart is the case in point: 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.