Master Guide: Tachograph Files

That .DDD or .TGD file you pull off your card or the vehicle unit is, these days, your best defence at a roadside check or a DVSA audit. Here's how to download it, open it, keep it and what to do when it throws an error. No jargon for the sake of it.

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Your tachograph file is legal evidence — treat it like one

A digital tachograph file isn't just a log of hours: it's a document signed on the inside, one that neither you nor the operator can touch without it showing. That's why, in an inspection, the file speaks for you. Downloading it on time and storing it the way the law asks is what stands between you and a serious infringement. In this guide we take it in order: what types there are, how often to download, how to open them, and what to do when something goes wrong.

What file types you'll meet (.DDD, .TGD and the rest)

The law is the same across Europe (Regulation (EU) No 165/2014), but every download program and every country slaps a different name on the file. That's where the confusion comes from. Don't let the extension scare you: at heart there are two families, depending on where the data comes from. If what you need is to open one right now, jump straight to how to open digital tachograph files.

The ones from your card (.C1B / .TGD)

These come off your driver card: what you've driven, rested and worked over the last 28 days (up to 56 with the Gen2 smart tacho). The .TGD format is standard across several EU regions, and you can open a .TGD online with our reader without installing a thing.

The ones from the lorry (.V1B / .DDD)

These download from the vehicle unit (the tacho box, the "VU"). That's where the fine detail lives: speed second by second, motion sensor faults, tampering attempts, GNSS border crossings and workshop calibrations. For the technical detail of the format, see what the .DDD file is.

How often to download and how long to keep them

No room to slip here: miss a download deadline and you can be penalised for every period that's missing. Learn these three numbers, they're the ones that really matter:

  • Your driver card: download it at least every 28 days. Let it drift and you build up gaps nobody can reconcile later.
  • The vehicle unit: at least every 90 days. It's the one that cross-checks the odometer against what your card says.
  • Keeping them: at least one year, exactly as they came off (don't rename or touch the insides, or the signature breaks), ready for when the DVSA asks for them at a desk-based assessment.

That year of custody carries more weight than it looks (who's responsible, where they're held, what happens when you change employer). We break it down in the guide to legal custody of tachograph files. And if the problem is that your operator is blocking the download of your own data, there's what to do and what protects you.

Why you can't open a .DDD in Notepad

If you've ever tried opening a .DDD in Notepad or Excel, you'll have got a wall of symbols. That's normal: the file is sealed with a digital signature and you need a program that knows how to read that signature from the card or the vehicle. That's what TachoTools does under the bonnet: it takes that unreadable file and turns it into a visual dashboard where you see the gaps and the overspeeds at a glance. If you'd rather do it yourself step by step, we cover it in how to read a DDD file, and so nothing slips past you when checking, in how to analyse .DDD files without errors.

When things go wrong: the errors you ask about most

Most tachograph scares aren't about driving too long — they're the file not reading, the card erroring, or the sensor playing up. The good news is nearly all of them have a known fix. These are the three that come up most:

Common questions about files (FAQ)

Yes, as long as the binary content inside hasn't been altered. The digital signature protects the data, not the file name. That said, for tidy records in your custody software, keep the standard naming with the download date and the card ID.

A file with an invalid signature has no evidential value before the authorities. It usually happens from interrupted downloads or reader hardware faults. Catch it early by analysing the file before it's ever asked for in an inspection, so you can do a fresh, valid download.

Yes, it's handy for reading or handing over a paper summary. But mind you: the PDF is a picture for humans, it won't stand as the official proof; what the inspection wants is the original file with its signature. Step by step in convert a tachograph file to PDF.