The miles per gallon readings on the FCD on my 2004 manual tranny HCH varies widely from the traditional "miles divided by gallons" calculations. And it's not ALWAYS optimistic:

Here are my first 13 tanks:

FCD M/G variance

37.8 38.4 (0.6)
43.1 40.7 2.4
45.0 42.9 2.1
48.4 48.5 (0.1)
53.3 49.7 3.6
55.6 53.9 1.7
54.8 51.3 3.5
52.6 49.9 2.7
52.6 46.8 5.8
47.3 52.8 (5.5)
46.1 43.4 2.7
46.5 43.4 3.1
46.9 49.9 (3.0)

I am at a loss to explain the discrepancies also. It's only an AVERAGE of 1.43 miles per gallon difference between the FCD and the M/G, so it's not a MAJOR problem. But it is OFF.