When I got the car originally the MFD would always over-report economy slightly - display would read, say, 45MPG over a tank, and calculate from brimming to around 43-43.5.  Far more accurate than most cars I've ever owned.

 

For the last few months at least it has gone the other way - today, for example, the display reads 45.3 and calculates to over 47.  This seems to be pretty consistently the kind of overriding I'm seeing.

 

Anyone got any idea what could be going on here?  The only thing I can think of is less fuel actually being injected than the ECU is requesting - is that a possibility?  Any other ideas?  I'm running stock tyres to correct pressures, no modifications to suspension or ECU.

does it really matter? its only 5% out so seems pretty good to me

It doesn't really matter, but I'm a detail person and often wonder if such details signify some change of significance.

 

I've never come across anyone saying their computer under-reports economy, always the other way, so it seems a little weird.

The dashboard display is pure fiction on these cars - even the coolant thermometer doesn't read true temperature, but is programmed to stay central over a wide range. 

 

The mpg display cannot accommodate variations in the calorific value of the fuel (eg. winter vs summer fuel). Brim to brim is the only way - averaged over a number of tankfulls as the venting system makes it difficult to fill the tank completely.

 

rotodiesel.

I'll do my best to let my OCD ignore it then!

I thought about winter vs summer, but even that didn't make any sense to me - surely all it tracks is amount of fuel injected vs distance measured.  Unless it tries to be cleverer than that...