Hi, Kristi... I don't have a Prius or an Escape, but I'll tell you how I use the GH db to accentuate how I keep track of mine...
I use the GH figure as a 'total lifetime FE' stat. It's got every one of my gals / miles / mpg's up there, so as long as the calculated average is 'total miles / total gallons', I'm ok (Jason, if it's avg added to total avg, let me know - and please consider changing it, because the number will be skewed badly if it's 'avg' and not 'total') I believe that I did check this the first couple of tanks and it jived, so I think we're ok
Since 'lifetime' has now been taken care of, I have 3 other measuring devices left - Trip A, Trip B, and the instantaneous (which I configure to reset on refill). I use Trip B for any trip I like, regardless of duration. I usually calc my 'to work' MPG, my 'from work' mpg, and then on weekends, I keep track for the day. I'll also use it to demo to friends who ride in the car.
Trip A then becomes my 'tank' mpg. It's not foolproof - the most accurate is always mi / gal, as indicated by your odo and gas receipt, and that's what I report - but it's a good indicator, and lately, I've noticed that it's not off by more than about 0.2mpg from actual.
Now, because I'm *truly* anal-retentive (enough to know that that word is hyphenated...
) , I also record actual / trip A / instantaneous in the 'calendar / memo' portion of my navi unit. Can't tell you the purpose it serves, other than having history readily available, but it's there for the future. I do not calc or keep the lifetime avg in there, though - that exists only on GH, although I could reconstruct it if Jason lost his server data, etc
I believe some folks with the dual trips leave one for a perpetual, and only reset the other. I tried that for a couple of tanks, but...
a. Realized I'd lose my 'tank avg' (the most accurate one, anyway, compared to the instantaneous avg)
b. Realized that I'd have no easy way to see total tank miles (again, Trip A / B are more accurate than the Instant.)
c. Thought: "what happens when someone resets it, either accidentally, or through having the car serviced, etc..." I can see myself having a coronary over that, in direct correlation to the number of miles travelled w/o reset, and it's not worth it.
So I reset A/B on every fillup, leave A for the 'tank', and B for whatever I choose, and let GH calculate the lifetime. Seems to work for me