Tas, knew I could count on you for a helpful reply. Thanks. Someone else on tdiclub echoed your thoughts about the scan gauge using MAF readings for mpg calculation.
Air Intake Upgrades and MAF questions. - TDIClub Forums
I switched the MAF sensor to the one I received with the 3" MAF Housing I ordered from vwvortex classifieds (from VR6 gas engine). I didn't expect the sensor to come with the housing, but the guy threw one in for some reason. Figured it would be d.o.a. if he just threw it in, but thats what I am still running and the engine runs awesome.
New problem today though, is a P0102 CEL code on cold startup (MAF reading low). If I clear it with the scan gauge it doesn't come back until the next startup. The car is pulling crazy strong and idles super smooth. I like the way it runs a lot. Not at all sure how my mileage has been affected though. If I get a MAF low trouble code, then the sensor must be under-reporting a low air flow number, but the acceleration has increased! Thats cool.
Driving to SAE World congress in Detroit this morning, it read 109 mpg. Too bad its a completely fictitious # though, since I observed the fuel gauge decline noticeably once I made the full round-trip.