Last November I was about to take out a DAF XF for an overnight run to Carlisle. Did my walkaround, got to the front and clocked the calibration plate on the headboard. The sticker showed the last calibration in October two years prior. Technically within window — barely. We pulled the actual certificate from the folder and the next due date was November 22nd. I was taking it out on the 24th.
Two days over. In a fleet that does overnight tramping, nobody had flagged it. The TM didn't know. It wasn't a dishonest operation — it was just an admin gap. The vehicle should have been off the road for a calibration appointment before that date. It hadn't been. If I'd taken it and got pulled, we'd have been looking at a PG9 prohibition and a fixed penalty notice, plus a potential DVSA maintenance investigation that nobody wanted.
I didn't take the vehicle. We sorted the calibration the following week. But the experience stuck with me because it was such a simple thing to miss.
What calibration actually involves
Tachograph calibration is essentially a check and reset of the unit's accuracy. An approved tachograph workshop — a DVSA-approved centre — connects specialist equipment to the tachograph and checks that the unit is correctly recording speed, distance, and time against the vehicle's actual characteristics. They verify the tyre size, the gearbox ratio, the characteristic coefficient of the vehicle, and make sure the tachograph is reading all of those correctly.
The reason this matters: the tachograph's compliance record is only meaningful if the unit is accurate. A tachograph that's recording speed or distance incorrectly is producing records that don't reflect what actually happened on the road. That's useless for compliance purposes, and at a roadside check or a DVSA investigation, inaccurate records create problems regardless of whether the inaccuracy was deliberate.
After calibration the workshop installs a calibration plaque — the sticker you see on the headboard or on the A-pillar area of the cab — and updates the seals on the unit itself. The calibration plate shows the vehicle identification, the installation date, the tyre size used, the characteristic coefficient, and critically, the date of the calibration and when the next one is due.
The two-year interval — and when it starts
The standard calibration interval under the retained EU regulations is two years. Every 24 months, the tachograph needs to be recalibrated by an approved workshop. The clock starts from the date of the last calibration, not from the date the vehicle was registered.
There are also additional triggers for an immediate calibration — or check — outside the regular interval:
- The tachograph unit is replaced or repaired
- The vehicle's characteristic coefficient changes — new tyres of a different size, gearbox work, diff changes
- The calibration seal is broken or tampered with
- The unit shows signs of malfunction or obvious inaccuracy
The tyre size one catches operators more often than you'd think. A fleet swaps a vehicle's tyres to a different specification — slightly different rolling circumference — without recalibrating the tachograph. The unit is now recording distance against the wrong tyre size. The records are technically inaccurate. That's a calibration trigger, not just a maintenance note.
What DVSA looks at during a calibration check
At a roadside stop, the examiner can check calibration status from the tachograph unit itself — the calibration data is stored in the unit and readable on a download. They can also look at the headboard plate. If the next calibration due date has passed, or if the seals on the unit are broken or missing, that's a direct path to a PG9 prohibition.
A PG9 is an immediate prohibition — the vehicle cannot continue in service until the defect is rectified and the prohibition is cleared by a DVSA examiner or the operator provides satisfactory evidence of rectification. For a tachograph that's past calibration, the prohibition stays until calibration is carried out and documented.
A PG10 is a delayed prohibition — the vehicle can complete its current journey but must be taken off the road for rectification before it goes out again. Calibration infringements can go either way depending on how far over the deadline the vehicle is and whether DVSA considers the unit likely to be materially inaccurate.
Either way, an operator who gets a calibration prohibition at a roadside stop is also looking at it appearing in their OCRS. And depending on what else DVSA finds in the download — if the records for the period since calibration lapsed look suspicious — it can prompt a deeper look.
Approved workshops — and who can calibrate
Not every tachograph workshop is an approved calibration centre. The approval is DVSA-granted and specific. Fitting a tachograph or replacing a card reader is one thing. Performing a legal calibration is another. The workshop needs to hold the appropriate approval and use calibrated test equipment that is itself regularly checked.
A list of approved workshops is available on the DVSA website. If your operator is sending vehicles to a workshop that isn't on that list for calibrations, those calibrations may not be legally valid. The calibration plaque they fit won't be recognised at a roadside check. I've heard of two operators who found this out the hard way after using a local mechanic who claimed to do calibrations but wasn't actually approved.
Keeping track of the fleet calibration schedule
For a single-vehicle owner-driver, calibration is just a diary entry every two years. For a fleet — even a small one, say six to ten vehicles — you need a system. Vehicles going in and out of service, different calibration dates for each unit, potential mid-cycle triggers from tyre changes. It stacks up.
The most common approach I've seen in small fleets is a spreadsheet with each vehicle, its last calibration date, and its next due date. That's fine until someone forgets to update the spreadsheet after a tyre change, or until the person who maintains the spreadsheet leaves the business and nobody picks it up.
A transport manager I know in Coventry uses a whiteboard with the next three months of mandatory dates across the entire fleet — calibrations, PMI dates, MOT dates, driver CPC training dates. Analogue but visible. If it's on the whiteboard, nobody misses it. That's a reasonable approach for a small operation.
The analogue tachograph — calibration differences
Analogue tachographs have the same two-year calibration requirement. The process is similar — approved workshop, characteristic coefficient check, updated calibration plaque. The additional complication with analogue units is the calibration of the stylus and the chart mechanism. Analogue tachos are comparatively rare on modern vehicles, but there are still older trucks in service using them, and the compliance obligations are identical.
If you're driving a vehicle with an analogue tacho — less likely on a newer fleet but not impossible on older owner-driver kit or specialist vehicles — check the calibration plate exactly as you would on a digital unit. The headboard sticker carries the same information. The consequences of a lapsed calibration are the same.
Driver's responsibility vs operator's responsibility
Calibration is an operator obligation, not a driver obligation. The operator is responsible for ensuring the vehicle has a valid, current calibration. The driver is not required to certify the calibration — only to carry out a pre-trip inspection and report defects.
That said: checking the calibration plate takes about three seconds on the walkaround. It's on the headboard or the cab pillar. The date is printed clearly. If you spot that the calibration is out of date, you report it and refuse the vehicle until it's sorted. You don't take the vehicle hoping DVSA doesn't notice, because you're the one at the wheel when they do.
The PG9 goes against the vehicle and the operator's licence. But the roadside encounter, the delay, the inconvenience of having the vehicle prohibited at a check site on the M6 at two in the morning — that's your problem to deal with at the time, regardless of whose fault the paperwork gap was.
If you want a single place to see your vehicle's next calibration date alongside your download schedule and your hours, ShiftOwt keeps those records in one dashboard — £5.99/month for drivers, operator pricing on request.