I do mostly highway driving for work and with a Lonestar 4x4 with 3.21 gears I can average about 17.5 to 18mpg. This is with 35"s and a level, and also if I don't include remote starting.
I have etorque so I actually let it start/stop and with how long some red lights are, it does help. Also, I try to find the sweet spot of highway RPM to set my cruise to. If I set cruise to 80 it'll average 18. If I really try hard and drive as conservatively as possible to work i can get the average to 19. But with normal driving and a few chances to floor it from redlights, etc I can hang out in the 17.5mpg range for work commuting.
During weekends I tend to do lots more city driving and then that will average out to 15 or 16mpg just depending on how hard I drive.
As for 87 vs 89 gas, I definitely seem to get 1 to 2mpg better with 89. I'll switch to 87 for a few tanks every now and then, but engine noise definitely seems to increase and my mpg seems to suffer. It's not enough to justify the increase in price per gallon, but the engine noise on 87 alone makes the 89 worth it.