Drag. Recall, wind resistance is proportional to the square of velocity. The vast majority of energy goes into overcoming drag, even at regular highway speeds. Remember when we capped speed limits at 55mph during the '70s energy crisis? We did so because fuel efficiency really starts to drop off at faster speeds. 250mph is very, very far up this curve.

Wikipedia has a nice graph to illustrate: https://en.wikipedia.org/wiki/File:Chart_MPG_to_L-100km_v200...