Manufacturers quote fuel economy figures obtained from official tests. These used to be the NEDC fuel economy tests – which were relatively short (testing cars over a distance of fewer than seven miles), which meant that most PHEVs could complete the tests with the petrol engine barely kicking in. The WLTP fuel economy tests were introduced in 2018 and they're meant to be more representative of real-world conditions than the NEDC tests. The test is longer (around 14 miles) and PHEVs are made to complete it in various different states of charge (i.e. from a full to empty battery). The official MPG figure that is quoted is an average of the various different results.

This is more realistic than the older NEDC tests, but it's still skewed by a lot of the tests being completed under electric power. This isn't entirely unrepresentative of reality – if you charge a car at home and never travel more than 10 miles away from your house, a tank of petrol in a PHEV could theoretically last for years. At the opposite end of the scale, if you cover a lot of long journeys and never charge it, the petrol engine will be running almost all the time and you'll see appalling fuel economy.