55-65 is really optimal, perhaps 55-60 being the best, but if you do a tick over 70 mpg and you won't see much over 18.5 highway.

no different than most trucks...some cars may be able to get better numbers at 70-75 due to wind resistance and rolling resistance reductions compared to bigger vehicles. 55 mph was mandated back in the 70's to save gas. air resistance is an exponential thing...something like the force to maintain 150 mph is 8 times more than what it is to maintain 75 even though velocity only increased 100 percent. wind at 50 mph is practically nothing so 55 does save fuel.