1977 sea ray ( comodore )

I have a 1977 SRVHT, 233hp. Wonder what kind of MPG I should be getting? My gas gage wont go lower than 1/4 and no higher than 3/4 so I don't think it is working correctly. Probably a problem in the sending unit in the tank. Anyway would like to know about what I should be averaging for MPG...