MPG falling as fuel tank gets lighter
 

Due to my working within city, I hardly drive on highways so all my milege readings are city readings. Recently I noticed that the milege measurements at 3/4 full tank and 1/4 full tank were different. When I fill up the tank with 3/4 full and 1/4 of full tank left are very different. At 3/4 full, my city mpg is at 16mpg however when I fill up the tank with 1/4 of fuel left, the milege is at 14 or less. This happens repeatedly. Could this be the sign of fuel pump dying?