How Much Can a Speedometer Be Out: Causes, Checks & Fixes

On this page, how much can a speedometer be out is the case in point: A speedometer that reads wrong has a cause, and the cause decides the fix. The most common cause is a tyre diameter change: the dial was calibrated for the original size, and a larger tyre makes it read low while a smaller tyre makes it read high, by exactly the diameter percentage. Other causes are wear and pressure, a previous recalibration for a different size, and faults in the sensor or the instrument cluster. This page works through the diagnosis — how to measure the error, how to tell a calibration error from a fault, and what to do about each — with a worked example using 205/55R16 (631.9 mm) and 215/55R16 (642.9 mm), a +1.74% change.

On this page, how much can a speedometer be out is the case in point: The first step is always measurement: compare the dial with GPS speed at a steady speed on a straight road, and work out the percentage from the tyre sizes. If the error is the same percentage at every speed, it is a calibration error and the formula on this page predicts it exactly. If the needle jumps, sticks or drops to zero, it is a fault and the vehicle needs diagnosis rather than recalibration. The sections below cover the formula, the tables, the common causes in detail, the GPS check and the fixes — with the disclaimer below applying throughout.

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 much can a speedometer be out: How the speedometer error is calculated

On this page, how much can a speedometer be out is the case in point: 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.

how much can a speedometer be out: What the speedometer actually measures

For how much can a speedometer be out, the point is this: The speedometer does not measure speed directly. It counts wheel revolutions — from a sensor on the gearbox output, a wheel-speed sensor or the ABS system, depending on the vehicle — and multiplies by the rolling circumference the manufacturer programmed in. That circumference belongs to the original tyre size, so fitting a different diameter changes the distance covered per revolution while the speedometer keeps using the old figure. The result is a fixed percentage error: if the new tyre is 2% larger in diameter, the actual speed is 2% higher than the indicated speed at every speed, and the odometer under-records by the same 2%. The error does not grow or shrink with speed; it is the same percentage at 30 mph and at 70 mph, which is what the tables below show.

how much can a speedometer be out: Worked example: 205/55R16 to 215/55R16

For how much can a speedometer be out, 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.

how much can a speedometer be out: Indicated vs actual speed: the worked example

On this page, how much can a speedometer be out is 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

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 much can a speedometer be out.

how much can a speedometer be out: Common causes of a wrong speedometer

Taking how much can a speedometer be out 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 much can a speedometer be out: How to fix a speedometer error

Taking how much can a speedometer be out as 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.

how much can a speedometer be out: Common mistakes with speedometer error

On this page, how much can a speedometer be out is 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.

how much can a speedometer be out: Related pages

Frequently asked questions

How do I calculate speedometer error for how much can a speedometer be out?

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 how much can a speedometer be out.

How much can a speedometer be out?

Taking how much can a speedometer be out 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?

On this page, how much can a speedometer be out is 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?

On this page, how much can a speedometer be out 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, how much can a speedometer be out 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.