Speedometer Error After Regearing: Error After Bigger Tyres & How to Fix It

On this page, speedometer error after regearing is the case in point: A lift kit usually comes with bigger tyres, and the bigger tyres are what put the speedometer out: the lift itself does not change the speedometer, because the speedometer counts wheel revolutions, but the larger diameter means each revolution covers more ground, so the vehicle travels faster than the dial shows. A 265/70R17 tyre measures 802.8 mm against a 235/65R17 at 737.3 mm, an 8.89% increase, which is well outside the ±3% guideline and large enough to matter for speed limits and the odometer. This page works the error out with the formula, shows the indicated-versus-actual table, and covers the fixes — recalibration, regearing and the order in which to do them.

Taking speedometer error after regearing as the case in point: The worked example below starts from the same pair — 235/65R17 to 265/70R17, +8.89% — and shows that at an indicated 70 mph the actual speed is 76.2 mph, a difference that would put the vehicle over a 70 mph limit while the dial shows exactly 70. That is why a speedometer correction is part of a lift-and-tyre job rather than an afterthought, and why the odometer and the gearing should be checked at the same time. The rest of the page explains the formula, the recalibration methods, the gear-ratio side of the change and the mistakes that leave the dial wrong after the work is done.

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 after regearing: How the speedometer error is calculated

For speedometer error after regearing, the point is this: 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.

What the speedometer actually measures for speedometer error after regearing

Taking speedometer error after regearing 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.

speedometer error after regearing: Worked example: 205/55R16 to 215/55R16

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. That is the point this page works through for speedometer error after regearing.

Indicated vs actual speed: the worked example for speedometer error after regearing

For speedometer error after regearing, 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 error after regearing.

speedometer error after regearing: Common causes of a wrong speedometer

On this page, speedometer error after regearing is the case in point: Tyre size is the most common cause of a speedometer error, but it is not the only one. Worn tyres have a smaller diameter than new ones, so a speedometer calibrated for new tyres reads slightly high as the tread wears — the effect is small, typically well under 1%, but it is real. Under-inflation reduces the effective rolling radius and has the same direction of effect, while over-inflation increases it. A different final-drive ratio does not change the speedometer reading itself, because the sensor measures wheel speed rather than engine speed, but a speedometer drive gear change or a recalibration done for a previous tyre size will leave the dial wrong for the tyres now fitted. And on older vehicles a worn speedometer cable or a failing sensor can make the needle wander or stick, which is a fault rather than a calibration error and needs repair rather than recalibration.

How to fix a speedometer error for speedometer error after regearing

Taking speedometer error after regearing as the case in point: 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 after regearing: Common mistakes with speedometer error

Taking speedometer error after regearing as 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.

speedometer error after regearing: Related pages

Frequently asked questions

How do I calculate speedometer error for speedometer error after regearing?

For speedometer error after regearing, 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?

On this page, speedometer error after regearing is 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?

For speedometer error after regearing, the point is this: 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?

Taking speedometer error after regearing 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 after regearing.