Speedometer Gear Change Calculator: Formula, Worked Example & Speed Tables

Speedometer Gear 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.

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. That is the point this page works through for speedometer gear change calculator.

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.

speedometer gear change calculator: How the speedometer error is calculated

On this page, speedometer gear change calculator 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 speedometer gear 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 gear change calculator.

Worked example: 205/55R16 to 215/55R16 for speedometer gear change calculator

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. That is the point this page works through for speedometer gear change calculator.

speedometer gear 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 gear 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

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 speedometer gear change calculator.

speedometer gear change calculator: Common causes of a wrong speedometer

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. That is the point this page works through for speedometer gear change calculator.

How to fix a speedometer error for speedometer gear change calculator

On this page, speedometer gear change calculator is the case in point: The fix is chosen by vehicle, not by preference. Modern vehicles with electronic speedometers are usually corrected by reprogramming the tyre-size setting with a diagnostic tool or a handheld programmer, which adjusts the speedometer and the odometer together. Older vehicles with a cable-driven speedometer need a different speedometer drive gear, and the new gear is selected using the diameter ratio — new teeth = old teeth × (old diameter ÷ new diameter) — so the gear compensates exactly for the size change. Where neither is available, an inline signal calibrator fitted to the speed sensor wiring can scale the reading. A dealer may be needed where the setting is locked behind manufacturer software. In every case, measure the error against GPS before and after the fix, at a steady speed on a straight road, to confirm the dial is right.

speedometer gear change calculator: Common mistakes with speedometer error

On this page, speedometer gear change calculator is the case in point: 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.

speedometer gear change calculator: Related pages

Frequently asked questions

How do I calculate speedometer error for speedometer gear change calculator?

For speedometer gear change calculator, the point is this: 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 speedometer gear change calculator 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?

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 gear 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 gear change calculator.

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 speedometer gear change calculator.