A weekend ↓ architecture v1.0 · 2026-08-11

The trip log that writes itself: stop guessing the business share of your vehicle

Every trip recorded automatically on a box you own, tagged in two minutes a week, exported as a year-end kilometre record

Setup: A weekend (~5-10 h) Running cost: $2-15/mo (software included) Time saved: saves ~0.2-0.5 h/week for the owner at one vehicle and 15-25 trips a week — basis After setup: You run it — records by itself; you spend about five minutes a week saying which trips were work Data lives: Your server — Positions, trips and the tagging table all stay on your box. The tracker's SIM carries data over the mobile network straight to your address, so no vendor holds a map of where your vans have been.

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Built for small businesses in general — not a sector list.

Before you start

A tracker per vehicle that speaks one of Traccar's protocols and can be pointed at your own server address, plus a data SIM: roughly EUR 65-125 one-off and EUR 1-3 a month each, prices move
A machine that stays on, reachable from the internet over HTTPS, because the tracker has to deliver positions somewhere
The odometer reading at the start and end of each year, photographed, or the totals have nothing to reconcile against
Someone who tags last week's trips every week: this is the part that cannot be automated and the part that fails first
If anyone other than the owner drives: a written geolocation policy naming every purpose and retention period, almost certainly a data protection impact assessment, and works council information where you have one
An accountant who has told you which deduction method you are using, before you build the export

What breaks first FAILS SILENTLY

the thing in the vehicle: a dead SIM, a blown fuse, or a dongle someone unplugged to charge a phone. A tracker that stopped reporting looks exactly like a vehicle that did not move, and you find out in December. Alert when a vehicle has reported nothing on a working day.

Version history

v1.0 · 2026-08-11 — First release. Traccar as the trip recorder, Grist as the weekly tagging table, with the two hard boundaries stated up front: a German Fahrtenbuch cannot be built this way, and tracking staff vehicles is employee monitoring.

Verdict first, because this one has a hard boundary. If you are one person with one van, build it: a tracker, an evening, and you replace a subscription that runs €10 to €25 per vehicle per month. If staff drive the vehicles, build it, but read the pitfalls before you plug anything in — the Belgian data protection authority ruled on 25 March 2025 that recording around the clock is disproportionate, and the fix is a setting you can only make on a system you own. And if you are in Germany trying to replace the 1 % rule with a Fahrtenbuch, stop here. The courts want a closed book whose edits are visible inside the book itself. A system you can edit is not that, and no self-hosted setup is going to become one.

The architecture

the same flow as text

      [ OBD-II tracker in the vehicle ]  or  [ phone app, one-vehicle setups ]

              │  positions over the mobile network, straight to your address

              ▼

      [ Traccar — the trip recorder (self-hosted, one small box) ]

              ├─ detects stops and closes a trip: start time, end time, addresses, distance

              ├─ geofences named after customers and sites → most trips label themselves

              ├─ reads the odometer off the OBD port where the tracker supports it

              └─ records nothing outside the hours you configure

              ▼  n8n pulls new trips once a night

      [ Grist — the weekly tagging table ]

              ├─ business · commute · private, plus the reason on business trips

              └─ anything untagged after 7 days is chased, not forgotten

              ▼

      [ year-end export ] every trip, odometer at both ends, one business percentage

      Nothing here decides anything. It records, asks, totals and exports.

Three parts: something in the vehicle, a recorder you own, and a table where a human says what the trip was for. The third part is not automatable and pretending otherwise is how logbooks get rejected — only the driver knows whether Tuesday 14:20 to Ghent was a customer or a dentist.

The problem

A vehicle that is used for work and privately has a number attached to it: the share of kilometres that were business. That number decides how much VAT you recover and how much of the running cost you deduct. Most small businesses arrive at it by feel, once a year, in the accountant's office. It is the single largest unverified figure in a lot of small-company tax returns, and it is the easiest one to check — garages are required to note the odometer on service invoices, so the total kilometres are already on paper somewhere.

What the rules actually ask for. In Belgium, decision E.T. 119.650/3 of 11 December 2012 lets you establish professional use through a trip administration kept either by hand or, in its own words, in an automated way via an adapted GPS system. It lists the fields: date of the trip, start address, end address, kilometres per trip, total kilometres per day, plus the odometer reading at the start and end of the period. In the Netherlands the tax authority asks for the same shape plus make, model, plate, and a private-or-business marker per trip, with the 500 private kilometres a year threshold hanging off it. In the United States the IRS wants a contemporaneous record, and 2026 has a wrinkle: the business rate was 72,5 cents a mile from 1 January and 76 cents from 1 July, so the log has to be splittable at that date. Different tax offices, one data structure.

The money, for a Belgian passenger car. Three methods exist. The flat 35 % applies to every vehicle you own and locks you in for four years. The semi-flat formula puts private use at ((commute distance × 2 × 200) + 6.000) divided by total annual kilometres. The trip administration gives you your real number, capped at 50 % deduction for a passenger car. So the gap you are chasing is 15 percentage points on the VAT of the car and of every litre of fuel, tyre and repair. On a light commercial vehicle — a real one, not a car dressed as one — the 50 % cap does not apply at all: the trip administration can take you well past it, and the alternative flat rate is 85 % if the van is predominantly professional, 35 % if it is not. That is where the logbook pays for itself fastest.

Against what you would otherwise rent. Vimcar's Fahrtenbuch lists at €24,90 per vehicle per month excluding VAT, with a platform fee from €19,95 a month on the fleet product from the second year. Webfleet is quoted around $25 per vehicle per month. At the small end, MileIQ is $13,99 a month or $11,66 billed annually with 40 free drives, and Everlance gives 30 automatically detected trips a month free, $8,99 a month after that. Three vans on a fleet plan is roughly €1.100 a year. The build below is €0 of software, one tracker per vehicle at $75 to $135, a data SIM at a euro or two a month, and a small always-on box you probably already have.

One boundary before we start. Which method suits your vehicles, what you may deduct and how long you must keep the record are questions for your accountant and your tax office. This blueprint is the plumbing that produces the evidence, not advice on what to do with it.

Tool choices — and why

Traccar

(Apache-2.0, 7.572 stars, v6.14.5 on 18/06/2026, pushed 08/08/2026) is the recorder, and it wins on one boring thing: protocol coverage. It speaks around 200 device protocols across a few thousand tracker models, which means you are not married to a hardware vendor and a dead tracker is a €70 replacement rather than a migration. It closes trips on stop detection and gives you a Trips report with start and end time, start and end address, distance and duration — which is, field for field, what the Belgian decision asks for. It has been maintained continuously since 2012. The web frontend and the mobile client live in their own repos, both pushed on 10/08/2026.

The thing in the vehicle

Traccar's own recommended hardware is the Freematics ONE+, an OBD-II dongle: Model A at $75,00 and Model B with 4G and GNSS at $135,00 on the maker's store. Any tracker speaking a supported protocol works, and the cheap generic OBD dongles do the job, but check two things before buying — that it reports on 4G rather than a 2G network being switched off around you, and that you can point it at your own server address instead of a vendor platform. Hardwired trackers survive someone unplugging the dongle; OBD dongles you can move between vehicles in ten seconds. Prices move, so treat those two figures as a reference point, not a quote.

Phone instead of hardware, if it is your own car

Traccar Client (Apache-2.0, pushed 10/08/2026) and OwnTracks (Android EPL-1.0, 1.744 stars, pushed 08/08/2026, with the OwnTracks Recorder at 1.188 stars, pushed 10/08/2026) both push positions to a server you run. Honest limits: a phone tracks the person, not the vehicle, it stops when the battery dies or the operating system kills the app, and it cannot read an odometer. Fine for a sole trader who wants to start tonight for €0. Not fine as the evidence base for a fleet.

Grist

(Apache-2.0, 11.438 stars, pushed 10/08/2026) is the tagging table, and the reason it is not a spreadsheet is that a spreadsheet has no history. Grist keeps a document history, so when someone asks why a trip flipped from private to business in March you have an answer. If you already run NocoDB (64.489 stars, pushed 10/08/2026) for something else, use that instead — the shape of the table matters more than the brand.

n8n

(Sustainable Use Licence, fair-code, 200k stars, pushed 10/08/2026) does two small jobs: pull last night's trips out of Traccar's API into Grist, and chase anything still untagged after seven days. If you already run n8n for the late-invoice chaser or the e-invoice reader in this library, this is one more workflow on the same box. Read the licence once if you plan to sell this setup to clients, because that is where the terms bite.

What we did not pick

A language model reading the trip and guessing the purpose. The purpose is not in the data — it is in the driver's head, and a plausible guess in a tax record is worse than a blank, because a blank gets asked about and a guess gets believed. There is no model anywhere in this blueprint and that is deliberate. We also did not pick a commercial fleet platform, but be fair about why: at one or two vehicles the honest comparison is not "free versus €25", it is an evening of your time plus a box against a subscription, and the Traccar project will rent you a hosted tracking account from $9,95 a month if you would rather not run it. The self-hosted case gets strong at three vehicles, and it gets strong immediately if you have staff, for the reason in the next section.

Setup outline

1. Get the two numbers you already have. Last year's odometer at 1 January and 31 December, from service invoices or the dashboard photo you never took. Divide your best guess at business kilometres by the total. That number is what you are about to replace with a measured one, and the difference is your business case.
2. Install Traccar on a small always-on box, behind a reverse proxy with a certificate. Add the vehicle. What you will see when it works: the device shows as online and a position appears on the map within a minute of the engine starting.
3. Fit the tracker and drive a known route — somewhere and back, a distance you know. Compare Traccar's trip distance with the odometer. Expect a small gap; write it down. If the gap is more than a couple of percent, fix it now, not in December.
4. Set the recording window. If anyone other than you drives, configure the system so it does not record outside working hours at all — not "records but hides", does not record. This is the step people skip and it is the one that matters legally.
5. Draw geofences around your yard, your regular customers and your home. Traccar will then name most arrivals for you, and the weekly tagging turns from typing into clicking.
6. Build the Grist table: trip id, vehicle, date, start, end, kilometres, category, purpose, customer. Wire the nightly n8n pull. What you will see when it works: Monday morning the table holds every trip from the weekend, untagged.
7. Tag one week by hand and time yourself. If it takes more than five minutes, your geofences are not doing enough work — go back to step 5 rather than accepting the chore.
8. Add the seven-day chase and the year-end export: every trip listed, odometer at both ends, totals per category, one business percentage per vehicle. Then run it in parallel with whatever you do today for one quarter and show the output to your accountant before you rely on it.

Pitfalls — the real ones

Germany is a genuine no

The Bundesfinanzhof confirmed on 12 January 2024 (VI B 37/23) what it has held since 2005: a Fahrtenbuch must be kept in closed form, and a computer-generated file only qualifies if later changes are technically impossible or documented and visible in the file itself. The Finanzgericht Düsseldorf case behind it (24 November 2023, 3 K 1887/22) rejected a commercial product precisely because its edits lived in separate log files. A Traccar plus Grist setup fails that test by design. Use this in Germany for cost allocation, reimbursement and knowing where the van is — not as the record that replaces the 1 % rule.

If staff drive, this is employee monitoring

The Belgian DPA's decision 61/2025 of 25 March 2025 is worth reading in full if this applies to you. Short version: legitimate interest is an acceptable basis, and location, kilometres, start and end times, duration and stop times are all data you may process. But continuous registration is disproportionate — the system must be capable of not recording outside working hours, and it is not enough that the employer merely cannot see it. Letting the driver switch it off themselves is flagged as best practice, especially on flexible hours. The DPA's own guidance also rules out using it to check speed, to monitor one person permanently, to track staff representatives, or to collect anything during commuting and breaks, even to deter theft. Write a geolocation policy that states every purpose and the retention period; the 2025 case found breaches for a purpose that was real but unwritten, and for a data list that ended in "et cetera". A data protection impact assessment is very likely required. And note the irony worth the whole build: "do not record outside these hours" is a configuration you can actually enforce when the server is yours.

GPS distance and the odometer are two different rulers

One measures the polyline between fixes, the other counts wheel rotations. They will not agree, and tunnels, urban canyons and a tracker that missed the first minute of a trip all widen the gap. Record the odometer at the start and end of the year from the dashboard, reconcile the totals against it, and keep the photo. Garages write the odometer on service invoices, which means the tax office has a cross-check whether or not you do.

Tag within a week or do not bother

A trip you classify in December is a reconstruction. Germany's tax administration guidance (BMF, 4 April 2018) treats an automatic log as contemporaneous only if the purpose is entered within seven calendar days — a good rule everywhere, and the reason the chase in step 8 exists.

The commute is private

Home to the fixed workplace is not business use in the Belgian and Dutch treatments, and it is the most common way a well-built log produces a wrong percentage. Give it its own category from day one rather than discovering the distinction in year two.

2026 has two US mileage rates

72,5 cents a mile through 30 June and 76 cents from 1 July, per Announcement 2026-11. If your export sums the year in one number, it is wrong. Split at the date.

The tracker fails quietly

A dead SIM, a blown fuse, a dongle someone pulled out to plug in a phone charger: all of them look exactly like a vehicle that did not move. Alert when a vehicle has reported nothing on a working day. Missing weeks are what turns a logbook from evidence into an argument.

Do not let it settle a dispute you have not had

The temptation, once the data exists, is to use it for something it was not announced for — timesheets, a suspicion about one employee. That is exactly the purpose-limitation breach the Belgian regulator found. If you want it for performance monitoring, say so up front, or do not use it for that.

Verified repos

Traccar — Apache-2.0, 7.572 stars, release v6.14.5 18/06/2026, pushed 08/08/2026
Traccar web frontend — Apache-2.0, 1.152 stars, pushed 10/08/2026
Traccar Client — Apache-2.0, 166 stars, pushed 10/08/2026. The older per-platform clients (traccar-client-android, traccar-client-ios) were last pushed 07/06/2025 — use this one.
OwnTracks Android — EPL-1.0, 1.744 stars, pushed 08/08/2026 · OwnTracks Recorder — 1.188 stars, pushed 10/08/2026, licence not resolved to a standard identifier: read the LICENCE file
Grist — Apache-2.0, 11.438 stars, pushed 10/08/2026
NocoDB — 64.489 stars, pushed 10/08/2026, licence not resolved to a standard identifier: read the LICENCE file
n8n — Sustainable Use Licence (fair-code), 200k stars, pushed 10/08/2026

Repo facts checked 11/08/2026. Belgian trip-administration fields from decision E.T. 119.650/3 of 11/12/2012; Dutch fields from the Belastingdienst rittenregistratie page; German requirements from BFH 12/01/2024 (VI B 37/23), FG Düsseldorf 24/11/2023 (3 K 1887/22) and BMF 04/04/2018; US rates from IRS Notice 2026-10 and Announcement 2026-11; employee-monitoring points from Belgian DPA decision 61/2025 of 25/03/2025 and the authority's geolocation guidance. Hardware prices from the Freematics store, subscription prices from vendor list pages, all checked 11/08/2026 and all liable to change. None of this is tax or legal advice: confirm your method, your deduction and your monitoring obligations with your accountant and, where staff are involved, with a lawyer.

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