Refuels & Fuel Efficiency
Questions about recording refuels and calculating fuel efficiency
How does My Cars calculate fuel efficiency?
Fuel efficiency is calculated by dividing distance traveled by fuel consumed:
Example: 500 miles ÷ 20 gallons = 25 miles per gallon (MPG)
The app uses the fuel quantity from your current refuel (not the previous one) because this represents the actual fuel consumed during that distance.
What is a partial refuel?
A partial refuel is when you don’t fill up the tank completely. Mark your refuel as “partial” so the app knows to accumulate the fuel amounts until your next full tank.
Fuel efficiency will be calculated once the accumulated fuel exceeds your tank size threshold (set in refuel preferences).
What distance values can I enter for refuels?
You can choose between:
- Odometer reading: Total mileage on your car (e.g., 50,000 miles)
- Trip distance: Distance since your last entry (e.g., 350 miles)
Switch between these modes in Settings > Data Entry > Refuel distance.
On vehicles with two energy sources (plug-in hybrid, LPG, CNG), trip distance means the distance since the previous entry of any type — including a charge or a fill of the other fuel. Don’t measure it from your last fill of the same fuel, or the same kilometres get counted twice and your total odometer drifts above the real one. If in doubt, use odometer mode and let the app do the arithmetic.
Your efficiency is worked out separately, over the full distance since your last fill of that same fuel — see Electric, Hybrid & Alternative Fuels.
Can My Cars remember my fuel or charging price?
Yes. If you usually pay the same price — for example a fixed home or contract charging tariff — set Settings > Data Entry > Pre-fill last price to Full price. Every new refuel or charge then starts with the price you entered last time for the same vehicle and fuel type, so you only change it when the price changes.
The other options:
- Remove last digit (up to 4 digits): pre-fills the last price without its final digits, so you only type the ending (handy when prices change a little every time)
- Disabled: leaves the price field empty (the default)
In older app versions this setting is called Last price digits cut count and the options are numbers: 0 means Full price, and 1–4 remove that many digits.
Why is my fuel efficiency showing as 0 or incorrect?
Common reasons:
- Partial refuels: Efficiency isn’t calculated until you do a full tank
- Wrong distance mode: Check if you’re using odometer vs trip distance consistently
- First refuel: The app needs at least 2 refuels to calculate efficiency