Measuring mileage with the fill-to-fill method

The meter on the dashboard is a guess. Your own fuel and odometer entries tell the truth.

The method

1. Fill the tank completely. Note the odometer reading.
2. Drive as you normally do.
3. At the next fill, fill the tank completely again. Note the odometer and the litres that went in.
4. Mileage = kilometres driven divided by litres filled.

A worked example (made-up numbers)

The odometer shows 12,400 at the first fill and 12,860 at the second. You filled 31 litres. You drove 460 km, so the mileage is 460 / 31 = 14.8 km per litre. Enter each fill in the vehicle logbook and it works the km since the last entry and the km per litre for you.

Why a single reading can mislead

Filling by value or part-filling the tank makes the calculation wrong. The pump stopping early, tank angle and a hot day change the litres a little. Look at the average of several fills, not one.

Why mileage drops

• Under-inflated tyres and poor wheel alignment.
• A dirty air filter or old spark plugs or injectors that need service.
• Heavy loads, roof carriers and stop-and-go traffic.
• Fast driving, hard braking and long idling.
• Air conditioning in heavy use.

A sudden fall of more than ten percent from your average is a reason to check the vehicle. It can also be a fuel leak or theft in a fleet, so record every fill.

Using it for a fleet

Compare vehicles of the same kind. A truck that always uses more than its twin deserves a look. Do this monthly and keep the logbook backed up.

Common questions

Keep going

• Vehicle logbookOpen the register
• All guidesMore for small businesses
• Data safetyWhere records are kept

The registers keep their data in your own browser and send nothing to us. Names, numbers and prices in the examples are made up. No law, tax or fee is stated here: check local rules where they matter. Checked 14 November 2026.