Tyre Size Speedometer Chart: Error Percentages & Speed Tables

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

Taking tyre size speedometer chart as 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 tyre size speedometer chart

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

What the speedometer actually measures for tyre size speedometer chart

Taking tyre size speedometer chart 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.

tyre size speedometer chart: Worked example: 205/55R16 to 215/55R16

For tyre size speedometer 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 tyre size speedometer chart

For tyre size speedometer chart, the point is this: 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

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 tyre size speedometer chart.

tyre size speedometer chart: Common causes of a wrong speedometer

For tyre size speedometer chart, the point is this: The causes divide into calibration and faults, and the distinction decides the fix. Calibration causes are tyre diameter changes, wear and inflation pressure: all of them change the distance covered per wheel revolution, and all of them produce a steady percentage error that the formulas on this page can predict. Fault causes are a failing wheel-speed sensor, a damaged reluctor ring, a worn speedometer cable on older vehicles, or a fault in the instrument cluster itself — these usually make the reading jump, stick or drop to zero rather than sit a steady percentage out. If the error is the same percentage at every speed, it is calibration; if it changes with speed or the needle behaves erratically, it is a fault and the vehicle needs diagnosis rather than a recalibration.

How to fix a speedometer error for tyre size speedometer chart

Taking tyre size speedometer chart as the case in point: Fixing a speedometer error means telling the vehicle the new tyre diameter, and the method depends on the vehicle. On many modern cars a handheld programmer or a diagnostic tool can rewrite the tyre-size setting in the body or engine control module, and the speedometer and odometer then read correctly for the new size. On older vehicles with a mechanical speedometer, the fix is a different speedometer drive gear in the gearbox, chosen so the gear ratio compensates for the diameter change. Some vehicles need a dealer visit because the setting is locked behind manufacturer software, and some need an inline speedometer calibrator fitted in the sensor wiring. Whichever method is used, the correct one depends on the vehicle, so the owner's manual or a specialist should be consulted before buying a tool — and the error should be measured against GPS before and after, at a steady speed, to confirm the fix worked.

tyre size speedometer chart: Common mistakes with speedometer error

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

Related pages for tyre size speedometer chart

Frequently asked questions

How do I calculate speedometer error for tyre size speedometer chart?

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

How much can a speedometer be out?

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

Why is my speedometer wrong after new tyres?

Taking tyre size speedometer chart as the case in point: 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 tyre size speedometer chart.

Does tyre wear affect the speedometer?

For tyre size speedometer chart, the point is this: 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.