New sending unit and fuel gauge is off

I had my mechanic install (the second) fuel tank sending unit and my gas gauge works as such: Reads full for about 30 miles and then drops rapitly to 1/4 tank after about 70 miles of highway driving. When I fill at the 1/4 tank reading, the car still has 12 gallons of gas. Has anyone ever expirienced this problem? My mechanic does not seem to know what the problem is. (74 duster, 225) Thanks!