Most fuel gauges are simply ohmmeters that read the resistance in the voltage coming through the sender in the tank. The sender changes the resistance as the fuel level moves a float attached to it. The resistance the sender provides varies by model and manufacturer. Likewise gauges vary by model. If the sender in your new tank is not intended for the same resistance range as your gauge, you will get erroneous readings. You need to confirm the ohm range for for the sender and gauge to determine if that is the problem. .... If they are properly matched it could be a faulty gauge, faulty sender or poor connection in the wiring connecting them and to the ground. If you decide to start troubleshooting them, come back and we'll have some more info.
BTW, my new Mercury 115 has a gauge that displays the fuel used. I reset it to zero at every fill up and subtract the amount used from the tank capacity to know how much is left. It is very accurate.