There are so many variables to fuel mile:
I don't get as good fuel mileage when the canoe is on the racks.
Deduct a little if the bikes on on the over bed racks.
Hills are killers. I know you should get the mileage back going down the same mountain that sucked all of the gas out of your tank going up....but it just don't happen like that. I gear down, and still have to use the brakes some going down the mountain.....both of those lower fuel mileage.
Level roads are my friend.
But I don't get to those very often.
I try to drive way ahead, stay off the brakes as much as possible, avoid stop and go traffic areas, stay in overdrive if I can keep the transmission from shifting, keep the tires aired up, and leave the cruise control off. I also have a low profile camper which I think helps a bit.
I get around 10 mpg if the taking the canoe along. I got 9.1 on 1 leg of a spring trip when big time headwinds.....there were all sorts of high wind warnings out.
Without the canoe, I am disappointed if I get under 10.5 mpg. 10.5 to 11.0 mpg, with 11.0 to 11.5 on the occasion when I travel fairly level roads, lower speed limits, and stay out of towns.