Quick answer
To calculate fuel consumption, record miles traveled and gallons used. Divide miles by gallons for mpg, or use liters and kilometers for L/100km.
Detailed answer
A simple approach is:
1. Fill your tank and note the odometer reading.
2. Drive normally until the next fill-up.
3. Note how many gallons (or liters) go in.
4. Check your odometer again.
Subtract the initial odometer value from the new reading to get miles traveled. Then divide miles by gallons added for miles per gallon (mpg). For liters per 100 km, use (liters used ÷ kilometers traveled) × 100.
Repeating this process over several fill-ups yields a more accurate average, smoothing out variations in driving conditions or partial fills. Modern cars often have a trip computer that estimates real-time and average mpg, but manual tracking confirms its accuracy. This data helps you spot changes if your car’s efficiency suddenly dips—often a maintenance red flag.
