Yeah, i have an auto trans and my mechanical gauge is definately not linear in its response, so i guess the 376 may make sense. But the thing that gets me, is that for it to say 376 miles left on the tank, it has to have a stored or preset fuel economy (maybe the average from the donor vehicle?)
Gallons used it should tell you righ away (i would think) and economy it may take a while to collect data and calculate. maybe resetting the computer by disconnecting the battery for 10 minutes like you mentioned should work.
I have seen 21 mpg out of my truck once. I've seen 20 mpg 4 times. I'm averaging 16.5-17.5 right now, I'll hit 18 if I drive like an old man on long trips