Chris are you compensating for tires in your calculation? I believe your factor should be right around 4%, so (odo miles *1.04)/gallons=MPG.
I believe I'm right there with you, high 12's to low 13's city, get out on the road and can hit the 14's on a good day. That's with a similar build; 285 75 r16, and about everything possible bolted to it.
82 fj40, 99 uzj100, 06 gx470
05 525EXC, 01 XT225, 09 TTR125, 07 TTR90, 07 CRF50