I've been doing some "scientific" experiments with my mileage and here is what I have discovered:
Highway driving at 65-70mph and my gas mileage is 22.2mpg
Highway driving at 80+mph and my gas mileage is 18.7mpg
City driving (stop and go) my mileage is 17.5mpg
It would seem that the 2004 Bonneville GXP meets predicted mileage based on the above driving conditions. Further experiments will include "aggressive teen road rage" and "stop and go racing house-wife" to see if I can further impact my findings.
Sorry for the ill attempt at humor... I think I am having a bad reaction to the turkey and stuffing! Hope everyone had a great Thanksgiving.