I have thought about this before... but how accurate are these (when working correctly) ?, I mean what sensors do they use to obtain the MPG etc.... ?

My car is currently averaging 33.4 MPG at the moment, but say for arguments sake I had a big upgrade (new head, turbo, injectors etc...) would it still calculate the MPG correctly (ie the amount of fuel going through the now bigger injectors) or would it knock it all out?