Oh, I don't know. Eighteen highway is pretty thrifty compared to a bench stock 454, which gets 12 highway, 14 if you baby it!
I tell everyone that you have to remember, when the vx came out, gas was 1.12 a gallon. Who cared about mileage?! Its getting better than 1 hp per cubic inch of displacement, which is the bare minimum of acceptable performance in my mind (an I mean Bare! If honda and ferrari can get 100+ hp per liter and make it last, why can't someone else?) on a vehicle that's pushing 4000 pounds of steel and rubber.
In that view, 18 doesn't seem so bad!