Newbie has Fillup question

I filled yesterday for the 5th time. I had driven 303 miles and now have 1103 total. I last filled at 800 miles. The 4 previous fills, the car computer was within a mile or 2 of my calculated mileage at the pump. Actually my calculated was usually the slightly higher.

When I filled yesterday at 303 miles, my computer indicated the average to be 52.5mpg. I was very excited that I was going to get my first tank over 50mpg.

There were 3 bars left. I was expecting to put in under 5.8 gallons. My shock was that I put in a bit over 7.1, for a calculated milage of only 43mpg. Needless to say I was disappointed and concerned. I stopped filling when the pump automatically shut off as always. There was no spillage. The temperature yesterday was 45 degrees when I filled in the evening, though the high temperature for the day was 54 degrees. I think I have all the facts covered, other than it was a new gas station for me.

Now with 79 miles on the new tank, the guage still reads full.

I have 2 questions.......... 1.) Could the pump be wrong or my bladder stretched? 2.) How many miles is normally driven for the gas bar to go down 1 notch from full on a refill?

Thanks.

Cary in S.D.