Measure fuel economy with a log, not a single impression.

A fuel-economy claim needs both distance and fuel quantity. In kilometres and litres, fuel economy is distance divided by litres used; consumption in litres per 100 kilometres is 100 times litres divided by distance.

Use comparable fill records

A common manual approach records the distance between full fills and the fuel dispensed at the next fill. Keep receipts and odometer readings. Follow the vehicle's normal refuelling instructions; do not keep adding fuel beyond the appropriate pump stop to chase an exact level.

Keep the context

Record route type, unusual traffic, load, air-conditioning use and any maintenance changes. These are possible influences on a result, not corrections that turn an imperfect road test into a laboratory measurement.

Look across several intervals

For several complete intervals, divide the total distance by the total fuel quantity rather than simply averaging individual km/L figures. An illustrative 300 km using 25 litres is 12 km/L. It is an arithmetic example, not a result from our planned Lancer.

Publish the limitations

Pump shut-off, incomplete records, odometer error and changing driving conditions can affect an estimate. Repeat records are more informative than one fill. A manufacturer bulletin hosted by NHTSA describes a fill-to-fill method and recommends repeating it across several tanks to account for dispensing differences.