Generally, the slower the speed, the better the gas mileage. The largest resistance a vehicle sees at a steady speed is the aerodynamic resistance. Full size trucks have a large frontal area, not to mention a relatively poor aerodynamic profile. The amount of force necessary to maintain a speed if going to be:
F = K * V^2
where K is some constant related to the aerodynamics of the vehicle and V is the speed.
nissangirl74 wrote:18-19 is the mileage posted by Nissan on their website. Real world numbers are often different. IF you get that much, I would say you would have to have almost no extra weight other than a driver, no incline at all on the road, and be able to set the cruise control somewhere around 70. MPG tends to decrease after 70. As far as octane is concerned, just use what the owner's manual recommends. Using a higher grade than what is recommended will not make you get better mileage.
Weight will only make a significant difference in city or stop and go driving. At highway speeds, the change in overall friction from weight is going to be quite small. At worse, a huge load might cause the angle of attack to change slightly and may cause an increase in the K constant, but generally, it won't a make a noticeable difference for steady state speeds.
I usually get about 12-13 mpg in the Titan. The best long range mileage I saw was when my mom borrowed the truck to go to Yellowstone years ago. She got about 15.5 mpg. Not sure what kinds of speeds she saw, but she definitely doesn't do the 80-85 I usually do on the highway.