Was looking at my car details online and was interested to find out how they worked out mpg.

VCAcarfueldata.org.uk - DVLA Terms and Conditions of Use

Imperial combined fuel consumption (mpg):

The combined figure presented is for the urban and the extra-urban cycle together. It is therefore an average of the two other parts of the fuel consumption test, Urban and Extra-urban cycles, weighted by the distance covered in each part.

Imperial urban fuel consumption (mpg) (cold):

The urban test cycle is carried out in a laboratory at an ambient temperature of 20oC to 30oC on a rolling road from a cold start, i.e. the engine has not run for several hours. The cycle consists of a series of accelerations, steady speeds, decelerations and idling. Maximum speed is 31mph (50 km/h), average speed 12 mph (19 km/h) and the distance covered is 2.5 miles (4 km).

Imperial extra-urban fuel consumption (mpg):

The extra-urban cycle is conducted immediately following the urban cycle and consists roughly half steady-speed driving and the remainder accelerations, decelerations and some idling. Maximum speed is 75mph (120 km/h), average speed is 39mph (63 km/h) and the distance covered is 4.3 miles (7 km).

I work mine out by dividing the number of miles i do by the number of gallons i use. LOL

This is good, thanks!