Record the same details
Keep the date, odometer reading, fuel volume and total paid. Note whether the tank was filled consistently; partial fills make direct comparisons harder. Do not include fuel put into a different vehicle or container in the same calculation.
Choose the right units
UK miles per gallon uses an imperial gallon, which differs from a US gallon. Litres per 100 kilometres is another useful format. Always label the units when comparing a receipt calculation with a dashboard or online figure.
Look over time
One short interval may reflect a fill difference, weather or an unusual journey. Compare several records with similar conditions. Treat a substantial unexplained change as something to investigate rather than jumping from a number to a mechanical diagnosis.
Keep cost and consumption apart
A higher fuel price raises the bill even when consumption is unchanged. Record both the volume and price so the two effects are visible. Use the journey calculator for planning and your actual receipts for the ownership record.