gauges

I have bought the stewart warner 1 1/2" gauges to replace the existing gauges in my 72 dart.
My problem is that when I hook them to 12v, chassis ground and the sender they will act erratically.
For example; the oil pressure will read 60 lbs, the tranny temp is about a 1/4" off zero and the fuel gauge reads about 3/4 full.
I have run grounds everywhere with no difference at all. I have a ground strap from the engine to the firewall also.
Please help.:(