Any one have any ideas i just replaced my sending unit on my 496mag. The gauge came up to about 35 but was not staying steady on the gauge. We took it out on the water and got up to about 45to 50 mph, it then drops below the zero mark. The gauge is a Gaffrig. Anyone no how to test the gauge?

What kind of sender did you replace? What brand? With a lot of gauge companies you have to use their senders because they are ohm specific to that gauge. i.e. If you try using a Hardin marine sender with a Livorsi gauge you'll get no results or wacky results. To test the gauge disconnect the send wire from the sender and ground it, then turn your ignition on and if the gauge does nothing its bad, if it pings out then the gauge is fine and its your sender. Good luck!