Speedometer Error at 30 MPH: Actual Speed for Every Error Percentage

Speedometer error at 30 mph is a percentage of the indicated speed, not a fixed number of mph, so the same diameter change gives a different mph error at different speeds. This page shows the actual speed at an indicated 30 mph for every error percentage from −3% to +3%, with the formula that produces each figure and a worked example using real tyre sizes. The tables use the nominal new-tyre diameters computed from the size codes, and real tyres vary slightly by brand, model, wear and inflation pressure, so the figures are accurate to within a small margin.

As an example, a 205/55R16 tyre (631.9 mm) replaced by a 215/55R16 (642.9 mm) is a +1.74% change, so at an indicated 30 mph the actual speed is 30.5 mph. A 2% error at 30 mph is 0.6 mph, and a 3% error is 0.9 mph — small numbers at town speeds that grow into meaningful ones at motorway speeds. The rest of the page explains how the error is calculated, why most speedometers already read slightly high, what the law requires, 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.

speedometer error at 30 mph: How the speedometer error is calculated

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. That is the point this page works through for speedometer error at 30 mph.

What the speedometer actually measures for speedometer error at 30 mph

For speedometer error at 30 mph, 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.

Worked example: error percentages at 30 mph

Take the 205/55R16 (631.9 mm) to 215/55R16 (642.9 mm) change, which is +1.74%. At an indicated 30 mph the actual speed is 30 × (642.9 ÷ 631.9) = 30.5 mph. The same arithmetic gives the whole table below: multiply the indicated speed by 1 plus the error percentage. A −3% error gives 29.1 mph, a −1% error gives 29.7 mph, a +1% error gives 30.3 mph and a +3% error gives 30.9 mph. The mph difference grows with speed even though the percentage stays the same, which is why the same tyre change matters more on the motorway than in town.

Actual speed at an indicated 30 mph

Error % Actual speed Difference
-3% 29.1 mph -0.9 mph
-2% 29.4 mph -0.6 mph
-1% 29.7 mph -0.3 mph
+0% 30.0 mph +0.0 mph
+1% 30.3 mph +0.3 mph
+2% 30.6 mph +0.6 mph
+3% 30.9 mph +0.9 mph

For speedometer error at 30 mph, the point is this: Read the table from the error percentage, which comes from the two tyre diameters, and the actual speed follows directly. The table also shows the legal point in miniature: a negative error means the dial reads high and the car is going slower than shown, which is the safe direction, while a positive error means the dial reads low and the car is going faster than shown. In the UK the law requires that a speedometer never under-reads, so a positive error is the direction to watch. The odometer follows the same percentage, so a +2% error means the odometer records 2% fewer miles than the car covers.

speedometer error at 30 mph: Common causes of a wrong speedometer

On this page, speedometer error at 30 mph is 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 to fix a speedometer error for speedometer error at 30 mph

For speedometer error at 30 mph, the point is this: 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 at 30 mph: Common mistakes with speedometer error

On this page, speedometer error at 30 mph 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.

Related pages for speedometer error at 30 mph

Frequently asked questions

What is my actual speed at an indicated 30 mph with a 2% error?

With a +2% error the actual speed is 30.6 mph; with a −2% error it is 29.4 mph. The error is a percentage, so the difference in mph grows with speed — a 2% error is 0.6 mph at 30 mph. Find the error percentage from the two tyre diameters using the formula on this page.

How do I find my speedometer error percentage?

Taking speedometer error at 30 mph as the case in point: Divide the new tyre diameter by the old tyre diameter, subtract one and multiply by 100. For example, 215/55R16 (642.9 mm) replacing 205/55R16 (631.9 mm) is (642.9 − 631.9) ÷ 631.9 × 100 = +1.74%. Both diameters are nominal new-tyre figures computed from the size codes.

Is a speedometer error at 30 mph dangerous?

For speedometer error at 30 mph, the point is this: It depends on the direction. A dial that reads low — a positive error — means the car is travelling faster than the driver believes, which risks exceeding a speed limit. A dial that reads high means the car is travelling slower than shown. In the UK the law requires that a speedometer never under-reads, so a positive error is the direction to watch.

How do I check my speedometer against GPS?

For speedometer error at 30 mph, the point is this: 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?

Taking speedometer error at 30 mph as 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.