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

Speedometer error at 100 km/h is a percentage of the indicated speed, not a fixed number of km/h, so the same diameter change gives a different km/h error at different speeds. This page shows the actual speed at an indicated 100 km/h 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 km/h the actual speed is 101.7 km/h. A 2% error at 100 km/h is 2.0 km/h, and a 3% error is 3.0 km/h — 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 kmh: How the speedometer error is calculated

On this page, speedometer error at 100 kmh 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.

speedometer error at 100 kmh: What the speedometer actually measures

Taking speedometer error at 100 kmh as the case in point: Speedometer error is a percentage, not a fixed number of miles per hour, and that is the key to understanding it. The dial shows the speed the car would be doing if the original tyres were fitted, and the actual speed is the indicated speed multiplied by the ratio of the new diameter to the old diameter. A small diameter change gives a small error — a 1% change is 0.7 mph at an indicated 70 mph — while a large change gives an error that matters for speed limits and for the odometer. Because the error is proportional, the same table works at any speed once the percentage is known, and the worked examples in this article show how to find that percentage from the two tyre sizes.

Worked example: error percentages at 100 km/h

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

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

On this page, speedometer error at 100 kmh is 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.

Common causes of a wrong speedometer for speedometer error at 100 kmh

On this page, speedometer error at 100 kmh is the case in point: 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.

How to fix a speedometer error for speedometer error at 100 kmh

Taking speedometer error at 100 kmh as the case in point: There are four common ways to correct a speedometer after a tyre change, and the right one depends on the vehicle. A handheld programmer or diagnostic tool can change the programmed tyre size on many modern vehicles, which corrects both the speedometer and the odometer at once. A speedometer drive gear — a small gear in the gearbox — does the same job mechanically on older vehicles, and the replacement gear is chosen from the diameter ratio. An inline calibrator, wired into the speed sensor signal, can scale the signal on vehicles where neither of the above is possible. And a dealer recalibration uses manufacturer software to change the setting where it is locked to franchised tools. Before spending money on any of them, measure the error against GPS at a steady speed so the correction needed is known, and confirm the method suits the vehicle — the wrong tool for the vehicle wastes the money and leaves the dial wrong.

speedometer error at 100 kmh: Common mistakes with speedometer error

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

Related pages for speedometer error at 100 kmh

Frequently asked questions

What is my actual speed at an indicated 100 km/h with a 2% error?

With a +2% error the actual speed is 102.0 km/h; with a −2% error it is 98.0 km/h. The error is a percentage, so the difference in km/h grows with speed — a 2% error is 2.0 km/h at 100 km/h. Find the error percentage from the two tyre diameters using the formula on this page.

How do I find my speedometer error percentage?

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

Is a speedometer error at 100 km/h dangerous?

Taking speedometer error at 100 kmh 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?

Taking speedometer error at 100 kmh as 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?

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 error at 100 kmh.