@mathowie After a few fillups and calibrations, you'll find your new ScanGauge gives a more accurate results. But even if it's initially off by a few percentage points, the relative feedback is still useful for fuel usage feedback.
Here's My take on this whole issue. I have owned and driven many cars and trucks, both with Manual Transmissions and Automatics. If you are in control, ie manual shifting, you can get exceptional mileage. I averaged 23-25 combined mpg in a 2000 Corvette with the manual. I accomplished this by accelerating in 1st, 2nd briskly, but not fast just enough to stay away the 1-4 skip shift, then letting the engine rpm drop until I could get into 6th and lope at 40-45. I also used this technique in my 2003 C2500hd Duramax, a got 19-21mpg around Phoenix. The Blazer that I have listed currently has an 700-r4 automatic, I accelerate smoothly up to at least 45, so it can shift into 4 and lockup the torque-converter, if I don't get above 42 or so it will hang in third gear at 1750 rpm, but in 4th around 1270 rpm or so.
I forgot to finish my thought. Engines are more efficient operating under a load, Higher numerical gears. Keeping an engine rpm high and lightly loaded wastes fuel, a medium loaded engine in it's operating range will be more efficient. Their is no hard a fast rule that you can only shift into 5th after you get to 50mph, if your engine has enough torque, up-shift, it will save you fuel. One of the reasons I dislike Automatics, another is they take some HP to turn their oil pumps, and are generally heaver than their manual counterparts.