Gas Question

I just got a new Fuel pump in my 97 Astro Van. I was having problems before. Anyways I just bought the thing over xmas. My question is how much gas does a Astro Van burn. it seems to me like mine is eating gas like crazy. For example I fill it up and it is past full for a day and ahalf then on a short trip (say 5 miles) it will go from past full to almost 3/4 full and then bounce back and forth.

If I put it in park it goes back to past full but slowly trickles back down as you drive to about half between full and 3/4.

Is this normal any suggestions?

You should get 21-23 mpg on the highway and 17 or a little less around town.