the servo ratings differ from 4v to 6v.....big differance in oz. of torque...
example...
hitec 645mg
Operating Speed (4.8V): 0.24sec/60 degrees at no load
Operating Speed (6.0V): 0.20sec/60 degrees at no load
Stall Torque (4.8V): 106.93 oz/in. (7.7kg.cm)
Stall Torque (6.0V): 133.31 oz/in. (9.6kg.cm)
hitec 5645
Operating Speed (4.8V): 0.18sec/60 degrees at no load
Operating Speed (6.0V): 0.15sec/60 degrees at no load
Stall Torque (4.8V): 143oz/in. (10.3kg.cm)
Stall Torque (6.0V): 168 oz/in. (12.1kg.cm)
this was taken from the servocity website...here's the linky..
http://www.servocity.com
click on hitec servo's