When you reset the onboard computer is left with a problem. At what point does it start displaying a number - because you have to drive it a few miles before the number will become meaningful.
It has the same problem in calculating the range display.
The solutions are:
- start with a display such as dashes and run like this until enough data is average to give a meaningful number and then start displaying it.
- start by displaying numbers that are unrealistically all over the place - particularly if the driver is in urban stop/start or driving gently down some long hills.
- start by displaying a middling seed value and seed the averaging system so that looks sensible and then let the number drift gradually as the averaging system gets filled with real numbers.
Basically when you're seeing 40mpg after reset the MB designers have gone with the third option and chosen to seed the setup at 40mpg.
This will also be tied in with the range calculation and makes the initial reset range look sensible.