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I have five different fuel economy gadgets and they all have issues one way or another.
In the case of diesels and my unthrottled petrol engines, you will likely find that absolute accuracy is simply not possible via OBD instruments. Remember that a diesel is constantly changing it's AFR and this is not typically reported via OBD, so your gauge just guesses at it.
I believe some of the cheaper units may just assume stoich and rely on user calibration, which will be way off if you normally drive in the city then go on a long trip.
Having said that, a lot of TDI owners have SGII's and on my own TDI I've had good results with a calibrated SGII. SGII is expensive and in some ways basic, but it works really well.
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