Air is thinner at altitude. I get an mpg or two more in the inter mountain west than I do on the coast. Hot temps help as well.I have neighbor with a TRX, driven daily, towed, long road trips, tried to be conservative, etc., all the experiments of an enthusiasts / gear head / fab shop owner, and he's never seen 12 mpg in over 8,000 miles so far, and he's tried really hard to get 12 mpg or above, and he couldn't make it happen. Just saying.