Fuel gauge not reading properly

I recently had a new fuel pump installed in my vehicle at a local Ford dealership here in Chesapeake, VA. When I picked up my explorer from the dealer and went to get gas the fuel gauge read about 3/8 tank full. I always fill my tank and then when I get back in my vehicle the fuel gauge needle is just above full. (I note that the fuel gauge has always been accurate and that I have kept detailed records on my vehicle for years and keep every gas receipt for review and evaluation). This time when I got back in my explorer the fuel gauge was still on 3/8 full and slowly over the next 15-20 minutes finally got back to above the full mark on the fuel gauge. The fuel gauge is now consistently in error by about 1/4 tank. For example, my explorer has a 17.3-gallon tank capacity (per Ford specs) and now when the fuel gauge reads 3/4 full it takes 8 to 9 gallons to fill it back up (instead of what should be about 4 to 5 gallons).
-What should I do or say to the dealer?
-What are the possibilities of what could have happened?
Thank you,
Michael