I think you are looking on the "current trip" page. You have the "fuel economy" page that has the option for 25, 50 and 450 miles. Then you have "trip 1" and "trip 2", which are different from the "current trip" page. So you have 4 pages to look at (if you have them all set to show on your DIC). Current trip. Fuel economy (with 25, 50 and 450 mile options). Trip 1. And finally Trip 2.
I have grown to like the current trip page, but I always keep my default page the fuel economy page (the one with the option for 25, 50 and 450 miles).
I use trip 1 for the lifetime of the vehicle. Never reset that one. as I like to see what the lifetime mileage of the vehicle is while I own it. I use trip 2 page for my current tank of gas, I reset that page each time I fill up. That way I can figure the MPG myself. Since I always go with somewhat bigger wheels and tires, making the actual mileage driven off by 3 to 6% or so (depending on the size of tire I have on), I then figure how many miles I actually drove by multiplying what the trip 2 page says for miles by 1.04% (just using 1.04% as an example) or whatever the difference in revolutions per mile actually is between the stock tires and the new, bigger tires I put on.
In fact, I have my truck at the local shop right now, they are putting on 285/70R18 tires on 18x9 Fuel wheels tomorrow (along with the RC 1.5" front level and RC UCA's). So with these new tires, which will be 2" taller than what is on it stock, I will multiply by 1.06 to get my true miles driven. These new tires are 6% taller than the stock 32" Duratracs that came on the truck.