Speedometer Error at 100 MPH: Actual Speed for Every Error Percentage

Speedometer error at 100 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 100 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 100 mph the actual speed is 101.7 mph. A 2% error at 100 mph is 2.0 mph, and a 3% error is 3.0 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 100 mph: How the speedometer error is calculated

Taking speedometer error at 100 mph as 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.

speedometer error at 100 mph: What the speedometer actually measures

Taking speedometer error at 100 mph as the case in point: 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 100 mph

Take the 205/55R16 (631.9 mm) to 215/55R16 (642.9 mm) change, which is +1.74%. At an indicated 100 mph the actual speed is 100 × (642.9 ÷ 631.9) = 101.7 mph. The same arithmetic gives the whole table below: multiply the indicated speed by 1 plus the error percentage. A −3% error gives 97.0 mph, a −1% error gives 99.0 mph, a +1% error gives 101.0 mph and a +3% error gives 103.0 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 100 mph

Error % Actual speed Difference
-3% 97.0 mph -3.0 mph
-2% 98.0 mph -2.0 mph
-1% 99.0 mph -1.0 mph
+0% 100.0 mph +0.0 mph
+1% 101.0 mph +1.0 mph
+2% 102.0 mph +2.0 mph
+3% 103.0 mph +3.0 mph

Taking speedometer error at 100 mph as the case in point: 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 100 mph: 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 error at 100 mph.

speedometer error at 100 mph: How to fix a speedometer error

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. That is the point this page works through for speedometer error at 100 mph.

speedometer error at 100 mph: Common mistakes with speedometer error

On this page, speedometer error at 100 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.

speedometer error at 100 mph: Related pages

Frequently asked questions

What is my actual speed at an indicated 100 mph with a 2% error?

With a +2% error the actual speed is 102.0 mph; with a −2% error it is 98.0 mph. The error is a percentage, so the difference in mph grows with speed — a 2% error is 2.0 mph at 100 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 100 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 100 mph dangerous?

Taking speedometer error at 100 mph as the case in point: 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?

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 at 100 mph.

Does tyre wear affect the speedometer?

On this page, speedometer error at 100 mph 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.