If you sit in the car with the engine idling for about 5-10 minutes, you can see the mpg dropping down 0.1 at a time (depending on how long since you last reset the computer) which suggests it's pretty accurate or at least that it wants you to believe that it is!
If you reset the computer and the work out your own mpg between two fill ups, you can compare and get an idea of how good a job it's doing.
Lotus will know the technical answer to how it actually measures mpg
