[UFO]

You have to calibrate the distance first. Mine is almost nuts on, -1% is my calibration. You have to set the fuel to DIESEL1. [Edit: DIESELa]

You have identified the key problem with Scangauge and diesels. It measures the air and attempts to infer fuel consumption, which is just wrong. But if you drive in a consistent manner, you should be able to get close on a tank by tank basis. My mileage only varies by about 1mpg between measured and SG calculated each tank.

You should only have to make 1 calibrated fuel adjustment when you notify the SG of the fillup. Just give it the amount of the fill, that is enough. If you maintain your driving style, it will reflect your mileage within its capability.

Edit: I checked my SG, set the engine size to 1.9l (of course), and fuel is DIESELa.