Sunpro gas guage problem

[xaircav]

Untill I can afford the 5" guage set for my '53 5 window I'm using some sunpro guages to monitor the '62 283 under the hood. Everything works fine except the fuel guage. I hooked it up per the instructions using a sunpro tank sending unit (tank is a Classic Parts 20 gal.underbed). When I turn on the key the guage pegs to full and I know it is less than half full. Electrical is now 12 volts and I checked for good grounds. Same thing happens with the sending unit out of the tank manually raising and lowering the float arm.