Accuracy of fuel consumption on mfd

[mcmaddy]

Yes tuning boxes will affect MFD readout by about as much as 10%.

[I wanted a GTi]

Another thing to think about is the effect of having a tuning box fitted - these allegedly make the fuel computer more optimistic.

Never checked mine against it - I don't think I could have handled the disappointment!

I checked mine with the tuning box fitted and it was 

Can't remember what the exact figures were but it was out by a fair bit.

[mcmaddy]

The tuning box is fooling the car into thinking it's using less fuel as they don't connect to anything to do with fuel sensors etc.