it may be the type of speed signal you have. if it is inductive then it won't work well at slower speeds. If there is another speed sensor on the tailhousing that might work better, or if there is a conditioned signal coming out of the ecu or something. Got a schematic?
Worst case you can get a gear tooth sensor and point it at an axle bolt or something like that:
http://ecomodder.com/forum/showthrea...tml#post178690