New sending unit and fuel gauge is off

Sure, you have the wrong sender that has the wrong Ohm rating. Swap it back out for the "Other" sender. As stupid as that sounds, the gauge manufactures make senders in 2 Ohm ratings. Take a look, closely, like through Summitracing.com.

http://www.summitracing.com/hr/

Check out the product line and when your searching sending units, click the box to compare and notice the ohm differences.