So. To add input to this thread (?) I just checked my mileage/gas use records. 11-13mpg. Almost no matter what.
I have a big hill to climb to get to my camping/wheeling/skiing spots, so that has an impact. So depending on when and where I fill up/calculate makes a bit of difference. And how loaded down the truck is: 300 miles to Pismo and back last month, ~12-13. Various trips up into the Sierra this summer, 10-12. A month of random city driving, 11-13. Rubicon, 7, including to/from mileage.

Last year's camping trip to Baja, fully loaded, piled-high roof rack, 4 people, 12 on the interstate, 10 maybe off-pavement (my litres-to-gallons calcs might be off, but I doubt it). Soon they're going to start selling the "winter blend" here, which is going to cut my mileage a bit.
One time I had to drive I5 north into a wicked headwind. I got like 8mpg. That hurt.
None of this is really anything I actually worry about very much.