The computer is pretty darn close most of the time. I usually compare my gallons used versus what I really put in and most of the time it's within a few tenths. I usually get 19 around town and if I have much highway driving it goes up to low 20's as a weekly average. I've taken a couple of trips, one east of Denver and got 26.5 and the other west of Denver and got 26 (that was driving through South Park at around 90 for some 40 miles). Most driving in the mountains I've gotten low 20's. My wifes V6 gets a little worse in the mountains but alot better on the open road.
06 GT Legend Lime, Konig Beyonds, Hurst, JBA, Steeda stuff, MAC CAI, Sniper tuned, LT's, catted H pipe, VIS Racing CF heat extractor hood, Tokico D-Specs, Silver Horse Racing, LLR #0174
08 GT Performance White, Konig Beyonds, Legend Lime accents.