WordNet (r) 3.0 (2006):

speed up
    v 1: move faster; "The car accelerated" [syn: accelerate,
         speed up, speed, quicken] [ant: decelerate,
         retard, slow, slow down, slow up]
    2: cause to move faster; "He accelerated the car" [syn:
       accelerate, speed, speed up] [ant: decelerate, slow
       down]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "speed up":
   accelerate, bundle, bustle, crack on, crowd, dispatch, drive on,
   expedite, forward, gain ground, haste, hasten, hasten on, hie on,
   hurry, hurry along, hurry on, hurry up, hustle, hustle up,
   open the throttle, pick up speed, precipitate, press, push,
   push on, push through, put on, put on steam, quicken, race,
   railroad through, rev, rush, rush along, speed, speed along, spur,
   stampede, step on it, step up, urge, whip, whip along