We tend to think of engine power in terms of snappy starts. But that's really about power to weight ratio. So the heavier the vehicle, the more horses it takes to achieve the same acceleration.
The juice it takes to maintain highway speed is spent to overcome friction (tires, engine, bearings, etc) plus wind resistance. Friction tends to be linear, (eg twice as much at twice the speed), but wind resistance is non-linear, and at 30 the wind on the windshield is a factor, but at 60 it's the significant majority of the fuel you're consuming. Wind resistance, or more precisely, drag, increases with the square of the speed (twice as fast uses 4 times the gas to fight the wind resistance). An interesting aside is that going twice as fast also requires eight times the power... here's a single link with more than most will want to know about all this:
http://en.wikipedia.org/wiki/Drag_(physics)