bostelle said:
Also, why can't VW make a computer that really measures mileage accurately? Jim
They can, and they do; my MFD is within +/- 0.2 MPG of hand-calculated values, others have reported the same, and no one I can recall has reported more than a 1 MPG difference (to put that in perspective, that latter value would be about a 2.5% error...pretty trivial, and to my mind that small an error might be attributable to either the ECU
or the hand-calculator...for example, not every filling station pad is level).
In years past, the ECU had no way of actually knowing how much fuel was delivered with each pop of the injector, so it just used a generic value...hence the large errors on older cars. On the '09 TDI, the ECU critically needs to know how much fuel is injected, so there are pressure sensors in the glow plugs, plus other stuff, which allow an accurate determination of fuel injected. I forget all the details; Drivbiwire has discussed this, as I recall. Hence the '09 MFD display of MPG is pretty durn accurate. I double-check mine about every tank or two, and it is always spot-on.