Road trip on my Focus. 1.4k miles

[Demorphius]

I'm planning on taking a road trip from Miami to Columbia, MO next week.

My Focus has been giving me roughly 20/MPG city, and doing the math, gas-wise, it was going to be a rough kick in the twins. I got a tune-up and an oil change hoping to get better mileage, but the only difference I noticed was that it runs better (faster acceleration, higher speeds for each gear) but It pushed the bar to about 26/MPG (that's city miles, mind you. I rarely drive on the highway long enough to see a difference) and well.. Being so many miles, I've been wondering if this'll be a good idea after all.. What should I be expecting mileage wise?

Focus 2001 ZX3 3dr hatchback
1.4k+ one way.
One passenger, not much luggage (maybe one bag?)

[Demorphius]

First part of the trip is done. I got an average of 300 miles to the tank going a steady cruise controlled 80/mph for the majority of the trip. Asides from the car accelerating faster than usual, nothing happened to the car itself. Waiting for the return trip.