Tachograph Calibration: UK Rules, Intervals, the Plaque and What It Costs

Calibration is the least glamorous item on the compliance calendar and one of the easiest to be caught out on, because the interval is long enough to forget and the triggers in between are things nobody associates with the tachograph. A tyre change on a Friday can make Monday's driving illegal. This guide sets out what the calibration does, exactly when the law requires it, what the plaque has to say, and what happens at the roadside when it is missing.

What a calibration actually does

A tachograph measures distance and speed by counting pulses from the gearbox sensor. To turn pulses into kilometres it needs three numbers that are specific to the vehicle as it stands today:

• w, the characteristic coefficient of the vehicle: how many pulses the sensor produces per kilometre travelled.
• k, the constant of the recording equipment: the value the tachograph is set to, which must match w.
• l, the effective circumference of the driven wheels' tyres.

The approved centre measures w on a rolling road or a measured track, sets k to match, records l and the tyre size, checks the unit's speed and distance accuracy are inside the regulation's tolerances, sets the UTC time, enters the VIN and registration number, seals the connections and fixes the plaque. On a digital or smart unit the whole record is also written into the vehicle unit's memory, which is why a download of the technical data shows when the last calibration was and who did it.

Change any of the three numbers without telling the tachograph and every distance and speed it records afterwards is wrong. Wrong records are false records, and false records are a serious offence regardless of intent.

When calibration is legally required

The rules come from Regulation (EU) 165/2014 and its technical annex, carried into UK law after Brexit, and enforced through the Transport Act 1968. A tachograph must be calibrated:

1. On installation or activation. A new vehicle, or a new or replacement unit in an existing vehicle, including a Smart 2 retrofit. The two-year clock starts here.
2. At least every two years from the date on the plaque. Not from the date of the last inspection you happened to notice; from the plaque.
3. After any repair to the vehicle unit, the motion sensor or the wiring between them, and after any seal has been broken.
4. After a change of tyres that alters the effective circumference: a different size, and on some vehicles a different make or construction with a different rolling radius. A like-for-like replacement does not need it; a change from the size on the plaque does.
5. After a change of registration number. The VRN is part of the record on the plaque and in the unit. Cherished plates, re-registrations after a change of ownership and vehicles brought in from Northern Ireland or abroad all trigger it.
6. When the clock is more than 20 minutes out against UTC. Digital units drift slowly; a unit that has been disconnected from the battery for a long period can drift a lot.
7. After a change to w, which in practice means a change to the gearbox, the differential ratio or the sensor.

Two points that surprise people. First, the two-year period is a maximum, not a target: an operator who inspects vehicles every year and has a calibration done at every other inspection is fine, but one who books it at 24 months and is late by a week is not. Second, there is no grace period. A vehicle whose plaque says 14 September 2024 is out of calibration on 15 September 2026, whatever is in the diary.

Analogue tachographs

Vehicles first registered before May 2006 may still run analogue units in national work, though the retrofit deadlines have removed most of them from international transport. The analogue regime is a two-yearly inspection, at which the centre checks accuracy and seals and fits an inspection plaque, and a full recalibration every six years or after any of the triggers above. The chart-based records still have to be kept for the same periods as digital data; see how long to keep tachograph records.

The installation plaque: what to check on a walk-round

The plaque is a small label, usually on the B-pillar or near the tachograph, fixed so that it is destroyed if removed. It must show:

• the name, address or trading name and approval mark of the approved centre;
• the date of the calibration;
• the vehicle identification number and the registration number;
• the values of w and k, the tyre size and the effective circumference l;
• on smart tachographs, the details of the GNSS and DSRC facility where required.

If a tyre or registration change has been dealt with by a centre without a full recalibration, there will be a second plaque next to the first recording the change. Missing, illegible or peeling plaques are enough on their own for a prohibition, because without the plaque the examiner cannot check the unit against the vehicle. Adding "plaque legible, date within two years" to the PMI checklist costs nothing and catches most of the problems before DVSA does.

What happens at the roadside

Calibration is part of every DVSA roadside check that involves the tachograph. The examiner reads the plaque, checks the date, compares the registration and VIN, checks the seals, and on a digital unit can read the calibration record from the unit itself. An out-of-date calibration is an offence for which the operator is responsible; the driver can be issued a prohibition on the spot until it is rectified, which in practice means the vehicle goes nowhere until an approved centre has seen it.

The financial side is a graduated fixed penalty, currently up to £300 per offence, plus the cost of the recovery and the lost day. The bigger cost is the OCRS impact: tachograph offences weigh heavily in the roadside targeting score, so one missed calibration makes the next stop more likely. And because calibration is one of the operator licence undertakings in all but name, a pattern of them is the sort of thing that appears in a public inquiry brief.

Smart 2 retrofits and calibration

A retrofit is a new installation: the old unit comes out, the Smart 2 goes in, and the centre activates and calibrates it as if the vehicle were new. Three consequences:

• The two-year interval restarts from the retrofit date, not from the old plaque.
• The new plaque replaces the old one, and the old unit's data must be downloaded before it is removed and kept for the retention period. A retrofit done without a final download loses the last months of vehicle records.
• The GNSS and DSRC elements are checked as part of the calibration, so a retrofit by anyone other than an approved centre is not a lawful installation even if the unit works.

Our Smart tachograph 2 guide has the deadlines by vehicle type.

Cost and time

Approved centres price calibration as a fixed job. Expect roughly £100 to £200 for a digital or smart unit, more if the vehicle needs seals, a sensor or a plaque replaced, and about an hour on the rolling road. Most operators bundle it with a PMI so that the vehicle makes one trip. Ask for the calibration certificate and keep a photograph of the plaque in the vehicle file; the certificate is what an auditor asks for and the photograph is what settles a dispute about a date.

Keeping calibration off the infringement list

• Put the calibration due date in the same calendar as the PMI, the MOT and the tachograph download schedule, and treat it with the same seriousness. The vehicle unit stores the last calibration date in its technical data, and Fleetalyse reads it with every remote download from a CAN-connected tracker such as the FMC650 fitted with the tachograph cable, so the next due date is reported rather than remembered.
• Make "plaque present, legible, in date, matches VRN" a line on the driver's daily walk-round or on the PMI sheet.
• Tell the tyre supplier. A fitter who changes a size without a note to the transport office is the single most common cause of an unplanned calibration.
• Recalibrate before, not after, a cherished-plate transfer takes effect.
• Check the clock on the printout. A unit showing the wrong time by more than a few minutes is drifting towards the 20-minute trigger.

The calibration record, the download schedule and the infringement reporting all sit in one place on Fleetalyse, alongside the live map. The remote tachograph download page explains how the vehicle unit and driver card files are collected without anyone visiting the vehicle.

Frequently asked questions

How often does a tachograph need calibrating?

At least every two years for a digital or smart tachograph, measured from the date on the installation plaque, and sooner after a repair, a tyre size change, a registration change, a clock error of more than 20 minutes or a change to the vehicle's pulse rate. Analogue units are inspected every two years and recalibrated every six.

Do I need a tachograph calibration after changing tyres?

Only if the effective circumference changes: a different size, or a construction with a different rolling radius. Replacing tyres with the size shown on the plaque does not require it. If in doubt, the approved centre can compare the new tyre against the plaque without a full calibration.

Can any garage calibrate a tachograph?

No. Only a DVSA-approved tachograph centre can calibrate, seal and plaque a unit. A general workshop can replace a tyre or a battery but cannot lawfully touch the tachograph installation.

What does tachograph calibration cost?

Typically £100 to £200 for the calibration itself, taking about an hour, plus parts if seals, the sensor or the plaque need replacing.

What is the tachograph plaque?

The installation plaque is the tamper-evident label near the unit recording who calibrated it, when, the vehicle's VIN and registration, the tyre size and circumference, and the w and k values. Examiners check it at the roadside; a missing or unreadable plaque can lead to a prohibition on its own.