Guides

What belongs in a mileage log, and why each field matters

A mileage log is a boring document with one job: letting somebody else agree with your number without having to trust you. That is the whole design brief, and it explains every field in it.

The five fields that do the work

Strip a mileage log to the minimum and five columns survive. Everything else is decoration.

• The date. Not just for ordering — it is what ties a drive to a period, and periods are how anyone reviews a log.
• Where you started and where you finished. Addresses, not coordinates, wherever they can be resolved. A reviewer recognises a street; nobody recognises a latitude.
• The distance. Measured, not estimated. This is the number everything else multiplies.
• The purpose. Which job, which client, which employer. Without it a log is a list of drives rather than a claim.
• The rate. What a kilometre is worth for that purpose, so the arithmetic can be checked rather than believed.

If a log has those five, a reviewer can multiply distance by rate and land on the same total you did. That is the only test that matters.

Why the fuel price must be frozen at the day of the drive

Fuel prices move monthly. If an app stores only your current price and recalculates everything from it, then a price change in June quietly rewrites what January cost — and a log you already handed to someone stops matching the copy they hold.

The fix is to write the price onto each trip as it finishes. The drive keeps the price of its own day, permanently. Update the price for next month and last month does not move.

That has a consequence worth understanding: setting a rate today will not fill in trips you drove before it existed. Anything that changes history should be an explicit action that shows you what will change before it changes, not a silent side effect of editing a setting.

Purpose belongs to whoever pays, not to the car

It is tempting to attach a rate to a vehicle, because vehicles are the obvious thing to configure. But the rate is not a property of what you drive — it is a property of who is paying. One car can serve two employers at two rates; a rate on the vehicle cannot express that without inventing a second, fake car.

Attaching the rate to the purpose instead makes the common cases fall out naturally. A commute an employer covers, a client that pays more than another, a personal drive with no rate at all. The vehicle is still a sensible fallback, because some rates genuinely do differ by vehicle type — but it is the fallback, not the source.

CSV or PDF: they are not the same document

A CSV is for someone who is going to compute with it. It opens straight in Excel or Google Sheets, the numbers stay numbers, and totals can be re-derived. It is the right file for an accountant or a finance system.

A PDF is for someone who is going to read it. Fixed layout, page numbers, totals at the end, the same on every screen it opens on. It is the right file for a manager approving an expense.

Two small details decide whether a CSV survives the trip. The field separator has to avoid colliding with the decimal separator — in a comma-decimal locale the columns must be separated by semicolons, or every row splits in the wrong places. And the digits themselves must be plain ASCII, because a spreadsheet cannot add up numerals it does not recognise as numerals.

What a mileage log is not

It is a record, not a ruling. Rates differ by country, by employer and by year, and what a given rate entitles anyone to is a question for the person or organisation paying — or for a qualified adviser. A logging app should keep an accurate, verifiable record and stay out of that conversation entirely.

That is why Speedometer One ships no official rates for any country. You set the rate, the app records what you drove, and the document says on its face that the figures come from you.

How Speedometer One does it

Trips record themselves as you drive, with the route, both addresses and the measured distance. Add a vehicle with its consumption and fuel price and each trip carries its own cost, frozen at that day’s price. Tag a drive with who pays, give the tag a rate, and the trip shows what it is worth — and which tag the rate came from. Then pick any period and export it as a CSV or a PDF.

If you want the numbers behind a single drive first, trip stats explained covers what each one measures.