A fuel-economy number is only as trustworthy as the fill-ups behind it. The useful method is pleasantly old-fashioned: record the odometer, fill the tank to the same sensible point, drive normally, and divide distance by fuel added.
The calculation in plain numbers
Distance economy = distance travelled ÷ fuel used. Metric consumption = litres used ÷ kilometres travelled × 100.
Worked example from the road
If the odometer shows 600 km between full tanks and the next fill takes 48 litres, km/L is 600 ÷ 48 = 12.5. L/100 km is 48 ÷ 600 × 100 = 8.0. For US MPG, convert the 600 km to 372.82 miles and the 48 litres to 12.68 US gallons: 372.82 ÷ 12.68 is about 29.4 MPG.
Why the result can move
Repeat the measurement over several tanks and use the total distance divided by total fuel. That smooths out a difficult week, an automatic pump shut-off that clicked early, or a holiday trip with a roof rack. Note fuel type, tyre pressure, passenger load and route if you are comparing changes after a repair.
A mistake worth catching early
The dashboard estimate is useful for trends but not always for accounting. Resetting the trip meter late, comparing a partial fill with a full fill, or assuming the tank capacity is the amount consumed will all distort the result. Fuel used is what you actually add, not the tank’s advertised size.
How a careful driver would use it
Start with measurements you can verify: a fuel receipt, the handbook, a confirmed tariff, the door placard or the lender’s written quotation. Then write the unit next to each number. This small habit prevents many errors because 30 MPG, 30 km/L and 30 L/100 km describe very different realities.
Keep the displayed answer rounded, but retain the unrounded calculation internally. If a route, vehicle or price changes, edit that input only and compare the new result with the old one. That makes the calculator useful for decisions rather than just a one-off number.
What to record with the answer
Save the date, vehicle or route, the original measurement and the unit standard. That detail sounds fussy until you revisit an estimate months later and cannot remember whether the price was per litre, US gallon or Imperial gallon. It is especially helpful when several people are sharing a trip budget or when a workshop is comparing before-and-after figures.
Do not hide uncertainty behind extra decimal places. Showing 58.33 is useful when the inputs are known; showing 58.333333 does not make a fuel price or laboratory economy figure more precise. Round for the reader, retain precision in the calculation, and explain which assumptions are likely to move first.
A note on measured versus quoted figures
Advertised specifications are valuable starting points, but measured results belong to the conditions in which they were collected. A vehicle’s test economy, battery range, kerb weight or power rating may use a defined procedure that is not the same as an owner’s commute. Keep the source label with the number instead of silently treating every figure as interchangeable.
When a result influences safety, legality or a large purchase, stop at the estimate and check the authoritative document. The calculator helps you understand the relationship and prepare a better question; it does not replace the placard, handbook, inspection report or signed finance disclosure.
One detail worth carrying forward
Fuel logs become especially useful after maintenance. Record the baseline before changing tyres, filters, alignment or driving habits, then measure again over comparable routes. One good tank proves very little; a pattern across several tanks can show whether the change was real.
Record your next two fill-ups and enter them in the MPG or L/100km calculator to check the arithmetic. For official specifications, safety limits and contract terms, use the responsible manufacturer, professional or lender document as the final authority.
Questions drivers often ask
Should I calculate from a full tank?
Yes. Full-to-full measurements are usually the simplest way to pair distance with fuel volume.
Why did economy improve after a longer trip?
Warm engines and steady speeds usually waste less fuel than repeated short cold-start journeys.
Can tyre pressure affect the calculation?
Yes. Under-inflation can increase rolling resistance, so check pressures cold before comparing tanks.
Updated September 10, 2026