89 caravan mileage low

So, I recently installed a new fuel pump on my van. Well, I thought my mileage was bad before. I was wrong. It's worse now. Before my fuel pump went out I was getting like 13 maybe 14 to the gallon. Now I'm getting like 8 to the gallon. Well, I can't figure it out. I installed correctly and I KNOW I did. I'm like 12,000 miles overdue for an oil change and my spark plugs and wires need to be changed. But like I said, my mileage was still a bit better before. Any ideas on why this might be going on?

Why don't you do a tune up? Seems silly to think your mileage will improve by changing the fuel pump. 12,000 miles overdue on the oil?!? You're trying to buy a new car, aren't you.