I guess I must be lucky because I just returned from vacation roadtrip to Colorado (from California) and I never got less than 20mpg even though the trip contained non-trivial off-road segments in San Juan Mountains (often 4wd low).
My numbers:
23.171
23.107
24.296
21.608
24.105 (offroad Ophir pass)
20.996 (offroad Imogene pass, Engineer pass)
20.014 (some dirt roads near Colorado Springs + Pikes Peak)
20.013 (offroad Cinnamon pass, California and Corkscreew gulch)
22.630
25.266 my best ever, 370 miles from Scipio, UT to Tonopah, NV (~65mph)
24.351
Offroading in San Juans was bliss and car deserves A grade. You can find more info about the trails here:
http://www.traildamage.com/trails/index.php
Just for completeness: 2008 Liberty Sport, skid plates, Firestone Destination AT tires, around 28K miles on it now.