DIM average mpg goes down while at stop light

[oragex]

I do reset it often, but like I said it doesn't always fluctuate, it doesn't seem related to being reset.

[DieselDoNicely]

Originally Posted by Orca2

Would have to agree,forget about mpg if it bothers you buy a Micra,lol.

Ill keep that in mind next week after another 700 mile. A drop to 40 mpg from the current 45 mpg would be a signal for getting the tools out.

I'm down the filling station for £100 of diesel every 6 working days.

DDN

[Bill_56]

Originally Posted by oragex

I do reset it often, but like I said it doesn't always fluctuate, it doesn't seem related to being reset.

The rate by which mpg drops when stationary at the traffic lights must be related to how recently it was reset, that's how averages work.

Quite apart from odd behaviour at traffic lights though, the 'average mpg' function is only of any real use if you let it run and run, for hundreds of miles, without reset. Whatever algorithm Volvo use for the 'average' does not seem to work well for short distances or single journeys. At best you'll get wildly inaccurate figures.

Mathematically and computationally 'average MPG' is a less trivial algorithm than meets the eye as it is usually meant to be a 'recent' average, so that it continues to react to changing conditions. Simply dividing distance since reset by fuel consumed would be easy and accurate, but wouldn't achieve the goal of reflecting 'recent' differences.

[Norfolk Jim]

I found it could be wildly inaccurate sometimes. Quite different compared to my Saab 9-5 which always seems spot on when filling up - I think Saab had this sorted.