Posting an update on my newish(700 miles on them) Mickey Thompson Baja Boss 295/60 R20's. My MPG is definitely down from the Nitto RG's of the same size. Granted, it is winter here now, so snow and ice to deal with, but even when not pulling my job trailer and just driving, my MPG's seem to be down a decent bit. 2 or 3 MPG (maybe even 4) less than the Nitto's.
Granted, the MT BB's are about 5 lbs heavier per tire, and they are also .4" taller (when both tires new, my Nitto RG's had very little tread left). The tread is deeper and bigger. And I kept my Nitto RG's at about 45 to 50 psi, these MT BB's at about 40 psi. But I think the extra weight, combined with the extra height is the culprit. Plus these tires grip/grab much better, so that could be causing part of this.
Don't get me wrong, I LOVE the tires. Such an upgrade for snow/ice/any terrain for traction over the Nitto RG's, that it is probably worth it to get worse MPG. Looks wise, the MT BB's have grown on me. I really like them for looks now also, whereas to start I liked the Nitto RG's more. Still hard to beat Nitto RG's for looks, but at this point I am very happy with the look of the MT BB's.