Speed Limiter Recalibration: When and Why It’s Needed

A speed limiter is calibrated at installation to ensure it limits the vehicle to the correct maximum speed — 56mph for HGVs or 62mph for buses and coaches. This calibration must remain accurate for the speed limiter to provide legal compliance and effective speed management.

Over time, or after certain types of mechanical work on the vehicle, the calibration can drift or become invalid. When this happens, the speed limiter must be recalibrated by an approved engineer. This article explains the triggers for recalibration, the process involved, and how to keep your fleet compliant.

How Speed Limiter Calibration Works

The speed limiter module determines the vehicle’s speed by measuring a signal from the vehicle’s speed sensor — typically a pulse-generating sensor on the gearbox output shaft, the rear axle, or data from the CAN bus network.

During initial calibration, the engineer establishes the precise relationship between the speed sensor signal and the vehicle’s actual road speed. This relationship depends on the gear ratios in the gearbox and differential, the rolling circumference of the driven tyres, and the characteristics of the speed sensor itself.

The calibration process involves driving the vehicle at a known speed (verified by a rolling road or calibrated GPS speedometer) and programming the speed limiter module to correlate its speed signal reading with the actual road speed. The set speed (56mph, 62mph, or whatever the operator requires) is then programmed as the limit.

Once calibrated, the speed limiter module is sealed with tamper-evident seals and a speed limiter plate is affixed showing the set speed, calibration date, and installer details.

When Recalibration Is Required

Recalibration is required whenever the relationship between the speed sensor signal and the actual road speed changes. The main triggers are:

1. Tyre Size Changes

If the vehicle’s driven-axle tyres are changed to a significantly different size or specification, the rolling circumference changes. A larger tyre covers more ground per revolution, meaning the vehicle travels faster for the same speed sensor pulse rate. A smaller tyre has the opposite effect.

Minor changes — such as replacing worn tyres with new tyres of the same specification — do not normally require recalibration, because the rolling circumference change is small (typically less than 2%). However, changing to a completely different tyre size, profile, or specification requires recalibration.

For fleets that operate with different tyre configurations at different times of year (for example, switching to winter tyres or all-terrain tyres), recalibration should be considered if the tyre rolling circumference changes by more than 3%.

2. Gearbox or Differential Work

Any mechanical work that changes the gear ratios in the driveline — gearbox rebuild, differential replacement, crown wheel and pinion change — alters the relationship between engine speed, driveline speed, and road speed. This requires recalibration.

Even if the replacement gearbox or differential has nominally the same ratios as the original, small manufacturing tolerances can introduce enough variation to affect speed limiter accuracy. Recalibration after driveline work is good practice and, for HGVs and PSVs, a compliance requirement.

3. Speed Sensor Replacement

If the speed sensor (or the CAN bus module that provides the speed signal) is replaced or relocated, recalibration is required. Different sensors may produce slightly different signal characteristics, and a relocated sensor may read from a different point in the driveline.

4. Tamper Seal Breakage

If a tamper-evident seal on the speed limiter is found to be broken — whether through deliberate tampering, accidental damage during other maintenance work, or natural deterioration — the vehicle must not be operated until the speed limiter has been inspected, recalibrated if necessary, and resealed.

A broken seal means the speed limiter configuration may have been altered. Even if no tampering has occurred, the absence of an intact seal means the speed limiter’s accuracy cannot be verified without recalibration.

5. Accuracy Concerns

If there is any reason to believe the speed limiter is not limiting at the correct speed — for example, driver reports, GPS data showing higher-than-expected speeds, or a failed speed check during a DVSA roadside inspection — recalibration should be carried out immediately.

6. Annual Testing

During the annual test (heavy vehicle MOT), the testing station will check the speed limiter on a roller dynamometer. If the speed limiter activates at a speed that is outside the acceptable tolerance (typically +/- 5 km/h of the set speed shown on the plate), the vehicle will fail the test. Recalibration will be required before a retest.

The Recalibration Process

Recalibration follows a similar process to initial calibration:

1. The engineer inspects the speed limiter module, wiring, and connections for any damage or fault conditions
2. The existing tamper-evident seals are removed
3. The vehicle is driven at a known reference speed (rolling road or GPS verification)
4. The speed limiter module is reprogrammed to match the current speed signal to the actual road speed
5. The set speed is verified
6. New tamper-evident seals are applied
7. The speed limiter plate is updated with the new calibration date and seal numbers
8. Documentation is completed (calibration certificate, seal record)

The recalibration process typically takes 30 to 60 minutes per vehicle, plus travel time if the engineer is attending your premises.

Maintaining Calibration Records

Operators must maintain records of every calibration and recalibration event. These records should include the date, the reason for recalibration, the engineer’s details, the verified set speed, and the new seal numbers. These records must be available for DVSA inspection.

AutoKontrol maintains central records of all installations and recalibrations performed by our approved engineers. If your original documentation is lost or damaged, we can provide replacement records.

Scheduling Recalibration

AutoKontrol’s Find & Fit service is available for recalibration as well as new installations. Our engineers can attend your premises to recalibrate individual vehicles or carry out batch recalibrations across your fleet.

Contact us to arrange a recalibration visit or to discuss a recalibration schedule for your fleet.