Well, in that case, I assume that they've worked out a "low energy consumption" flight plan. The speed they're moving is the best that they can maintain without (most likely) ending up dead in space. It's fair to say that real physics would apply here... suppose that we're talking about the "hybrid" system I'm a proponent of, where impulse is basically a thrust-based system, combined with a mass-reducing/local-c-increasing subspace field. While the "thrust" component of that would only be required to operate occasionally (for minor course corrections... after all, they'd be moving in what is essentially a straight line, adjusting for gravitational effect, wouldn't they?), they'd still need to keep the static subspace field operational at all times, or else they'd drop down to the "true sublight equivalent" of whatever velocity they were moving at within their "impulse subspace bubble." Thus, even without the need for thrust, the need to "burn fuel." In this case, hydrogen in the fusion reactors, presumably.