WordNet (r) 3.0 (2006):

synchromesh
    n 1: an automotive system for shifting gears in which the gears
         revolve at the same speed and so shift smoothly

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "synchromesh":
   Hydromatic, automatic transmission, cogwheel, differential,
   differential gear, freewheel, gear, gear train, gearbox, gearing,
   gearshift, gearwheel, high, intermediate, low, neutral, overdrive,
   rack, reverse, standard transmission, stick shift, transmission