So I fitted a tuning box to my 147 JTD, and was pleased to feel better response and power (although I've set it on 7/10 just because it feels nicer). I also seemed to get better mpg too. According to the computer I did 215m at 60mph average, and got 58.2mpg (if memory serves).
I included this in another post, and someone suggested that because the box sits between the ECU and the injectors that the mpg would be inaccurate- the fuel added doesn't match with what the ECU thinks happened.
So my question is this: does the computer monitor actual fuel consuption or what the ECU thinks it's used? My gut feeling is the actual consuption, but I really don't know as I'm still learning about this stuff.