I don't have the Scan Gauge. I just use the mpg readout on the NAV system. I reset it every fill up and usually wait until I'm on E to fill up. I've noticed that since I bought my current FEH (an '07 which I purchased in August of '06), the mpg readout has been getting more accurate. On the drive home from the dealership the first day, it said I got something like 47mpg for my roughly 15 mile city drive. That was definitely way too high. It started getting more and more accurate as I had more fillups, but sort of leveled off at being 2mpg higher that what I calculated each time. That is until my last 3 fillups. They have been spot on.

I owned an '05 FEH for about a year before I traded it in for my '07. It was spot on almost every time.

I think a major factor that it comes down to is when you stop pumping gas into your tank. Do you "top it off" after the first time the pump clicks off or not? I don't know the "correct" answer, but I would guess that being consistent (either always or never topping it off and using the same gas pump) would give you the most accurate results. As to whether that would help the hand calculations match up to the readout, I don't know.