Speedometer Error Tyre Size Change Calculator: Formula, Worked Example & Speed Tables

Speedometer Error Tyre Size Change Calculator is answered by one formula and two tyre diameters. Error % = (new diameter − old diameter) ÷ old diameter × 100, and actual speed = indicated speed × (new diameter ÷ old diameter). This page applies the formula to the question in the keyword with a worked example using real tyre sizes, tables at 30, 50 and 70 mph, and the practical consequences explained in plain terms. The worked example uses 205/55R16 (631.9 mm) replaced by 215/55R16 (642.9 mm), a +1.74% change, and the tables use the nominal new-tyre diameters computed from the size codes — real tyres vary slightly by brand, model, wear and inflation pressure.

Taking speedometer error tyre size change calculator as the case in point: The reason the error matters is that it is proportional: the same percentage applies at every speed and to the odometer as well, so a small percentage becomes a meaningful mph difference at motorway speed and a meaningful mileage difference over a year. A larger tyre makes the dial read low, so the car travels faster than the driver believes, and a smaller tyre makes it read high. The sections below show the arithmetic, the tables, the common causes of a wrong reading, how to check the dial against GPS, and when a recalibration is needed.

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 tyre size change calculator

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 tyre size change calculator.

What the speedometer actually measures for speedometer error tyre size change calculator

A speedometer measures how fast the wheels are turning and converts that into a road speed using the rolling circumference of the tyres it was calibrated for. Change the tyre diameter and the conversion is wrong by exactly the percentage the diameter changed: a tyre 3% larger in diameter covers 3% more ground per revolution, so the car travels 3% faster than the dial shows, and the odometer records 3% fewer miles than the car actually covers. A tyre 3% smaller does the opposite. The error is proportional at every speed, which is why the tables in this article show the same percentage at 30, 50 and 70 mph, and why the formula below is all that is needed to work out the error for any size change. That is the point this page works through for speedometer error tyre size change calculator.

speedometer error tyre size change calculator: Worked example: 205/55R16 to 215/55R16

Taking speedometer error tyre size change calculator as 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.

speedometer error tyre size change calculator: Indicated vs actual speed: the worked example

Error: +1.74%. Actual speed = indicated speed × (642.9 ÷ 631.9). That is the point this page works through for speedometer error tyre size change calculator.

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

Taking speedometer error tyre size change calculator as the case in point: 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.

Common causes of a wrong speedometer for speedometer error tyre size change calculator

On this page, speedometer error tyre size change calculator is the case in point: Tyre size is the most common cause of a speedometer error, but it is not the only one. Worn tyres have a smaller diameter than new ones, so a speedometer calibrated for new tyres reads slightly high as the tread wears — the effect is small, typically well under 1%, but it is real. Under-inflation reduces the effective rolling radius and has the same direction of effect, while over-inflation increases it. A different final-drive ratio does not change the speedometer reading itself, because the sensor measures wheel speed rather than engine speed, but a speedometer drive gear change or a recalibration done for a previous tyre size will leave the dial wrong for the tyres now fitted. And on older vehicles a worn speedometer cable or a failing sensor can make the needle wander or stick, which is a fault rather than a calibration error and needs repair rather than recalibration.

speedometer error tyre size change calculator: How to fix a speedometer error

On this page, speedometer error tyre size change calculator is 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.

speedometer error tyre size change calculator: Common mistakes with speedometer error

A few errors recur whenever speedometers are discussed. One is assuming the factory calibration was exact — it rarely is, because manufacturers deliberately calibrate dials to read slightly high, and in the UK the law requires that a speedometer never shows less than the true speed. Another is using a phone GPS reading taken while accelerating or cornering, when GPS speed is least reliable; the comparison should be made at a steady speed on a straight, level road. A third is forgetting the odometer: the same percentage error applies to recorded mileage, so a car that has run larger tyres for years has covered more miles than the odometer shows, which matters for service intervals and resale. And the last is assuming the fix is always a recalibration — sometimes the right answer is to return to a tyre size within the ±3% guideline and check the placard first. That is the point this page works through for speedometer error tyre size change calculator.

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Frequently asked questions

How do I calculate speedometer error for speedometer error tyre size change calculator?

Taking speedometer error tyre size change calculator 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?

On this page, speedometer error tyre size change calculator is 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?

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 tyre size change calculator.

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 speedometer error tyre size change calculator.

Does tyre wear affect the speedometer?

On this page, speedometer error tyre size change calculator 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.