If the B4 sounds bad accelerating and decelerating you probably have the gear mesh set wrong like a few people have told you already. The diff might be slipping or the gearbox might just be in need of a bit of TLC.
Here is the instruction manual for a speedo that has a similar setup routine to yours. Page two describes how to calibrate the speedo to your transmitter.
http://www.hobbywing.com/upload/manu...9_20080102.pdf
Follow what the manual said religiously and it ought to work. If it doesn't try the whole thing again with the throttle channel reversed.