Are you 2 or 4 WD?
If you drive over 70 MPH on highway, with 2WD, 24 MPG is probably about right for pure highway.
I have broken 34 MPG for 25 miles by keeping it around 55 MPH on a nice stretch of highway with V6 & 2WD. I have struggled to get 22 MPG for 25 miles at 75 - 80 MPH on flat highway. And of course, everything in between.
I can get almost 30 MPG on my commute of about 14 miles to work, driving home I can take that 30 MPG and see it average back down to 20-21 MPG because the traffic flow is so different, with a lot more traffic lights. Construction in Fort Worth is ridiculous.
I have a 4wd. My commute is 30 miles, about 20 straight highway where I'm generally doing 80 (75 will get you nasty looks). I guess a little less on the pedal might improve a little but it seems like some of you guys are overachievers. :)
2016 Colorado LT, Crew Cab
LT Convenience Package
Chevy (factory) Drop-in Bedliner
Black - Westin Pro Traxx 4" Oval Nerf Bars
TonnoPro Tri-fold Tonneau
AVS In-Channel Window Visors