Speedometer Over Reading Causes: Causes, Checks & Fixes

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. That is the point this page works through for speedometer over reading causes.

Taking speedometer over reading causes as 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 the speedometer error is calculated for speedometer over reading causes

On this page, speedometer over reading causes is the case in point: The arithmetic is short. First find the percentage change: error % = (new diameter − old diameter) ÷ old diameter × 100. Then find the actual speed: actual speed = indicated speed × (new diameter ÷ old diameter). Both formulas use the nominal new-tyre diameters from the size codes, and the tables in this article use those figures throughout. A positive result means the replacement tyre is larger, so the true speed is higher than the indicated speed and the odometer under-records; a negative result means the replacement is smaller, so the true speed is lower than the indicated speed and the odometer over-records. The same percentage applies at every speed, so one calculation covers the whole speed range.

What the speedometer actually measures for speedometer over reading causes

For speedometer over reading causes, the point is this: 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.

speedometer over reading causes: Worked example: 205/55R16 to 215/55R16

Taking speedometer over reading causes 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.

Indicated vs actual speed: the worked example for speedometer over reading causes

For speedometer over reading causes, 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 speedometer over reading causes.

Common causes of a wrong speedometer for speedometer over reading causes

For speedometer over reading causes, 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.

speedometer over reading causes: How to fix a speedometer error

For speedometer over reading causes, the point is this: 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.

Common mistakes with speedometer error for speedometer over reading causes

The most common mistake is assuming the speedometer was accurate before the tyre change. Most speedometers are deliberately calibrated to read slightly high — in the UK the law requires that a speedometer never under-reads, and it may over-read by up to 10% plus 4 km/h — so a car that read 2% high on the original tyres and now reads 2% low on larger tyres has actually changed by 4% from what the driver was used to. The second mistake is checking the speedometer against another car's speedometer instead of against GPS: two dials can share the same error. The third is testing at one speed only and assuming the error is a fixed number of mph — it is a percentage, so the mph error grows with speed. And the fourth is treating a calculation as a reason to ignore the limit: if the dial reads low, the car is travelling faster than the driver thinks, and that is the direction that risks a speeding offence. That is the point this page works through for speedometer over reading causes.

speedometer over reading causes: Related pages

Frequently asked questions

How do I calculate speedometer error for speedometer over reading causes?

For speedometer over reading causes, 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?

For speedometer over reading causes, 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 over reading causes.

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 over reading causes.

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 over reading causes.