There is a mode on our 04 Sienna le where it tells the temperature that shows your average gas milage and we assume that since it hasn't changed that it is the average for the cars lifetime. We didn't know if there was a way to reset it to determine the gas milage on long trips or not. If you know of any way please give your answer, I would much appreciate it.

try this. using the button to get to the gas mileage and stuff, hold it down when the average fuel mileage comes up. it should reset. that answer from that girl is stupid dont listen to her

. I've got a similar 'gauge' on my Lincoln, I think, and it changes ALL the time, like goes from 28 down to 21 when I pass somebody.
WALK in to your Toyota dealers Parts Dept and ask them. I'm sure there's no 'reset' button but maybe that mileage 'mode' is only for appearance? ..maybe a fuse?
Or you might want to go to a Toyota 'forum' and post the Q.