Can I use kilometers and liters per 100 kilometers? +
Only after conversion if the available entries use miles, MPG, and price per gallon. Convert the route to miles, convert L/100 km to MPG, and convert the fuel price to a per-gallon amount in the desired currency. Do not combine kilometers with MPG or a per-liter price, because those units do not produce a meaningful cost.
Does the yearly total predict what I will actually spend? +
No. It extrapolates the entered distance, frequency, MPG, and fuel price. Holidays, remote-work days, detours, seasonal fuel economy, vehicle changes, and price movements can all alter actual spending. Treat the result as a planning estimate, and rerun it when your commute pattern, observed MPG, or local fuel price changes.