An official fuel-economy figure is a laboratory result from a standard test cycle — it is perfect for comparing one car to another, and optimistic about what you'll actually get.
City, highway and combined
Most figures come in three flavours: city (stop-and-go), highway (steady speed) and combined (a weighted mix). The combined number is the one to compare across cars; the city and highway split tells you which suits your driving.
In the US these come from the EPA cycle. MotorHangar converts them to a common unit so you can line cars up side by side, and labels the source — real-world figures in your country depend on fuel, traffic and altitude.
Why your mileage is different
The test is gentle: no hills, no air conditioning fighting a heatwave, no roof box, no city gridlock. A useful rule of thumb is to expect 10–20% worse than the combined figure, more if you drive hard or short trips.