Bad Fuel economy

Thanks for replying Tech1. I should have mentioned that the fuel pump was replaced. It was replaced because the sensor that relays how much fuel is in the tank went bad. I was told that the sensor could not be replaced but the whole fuel pump must be replaced. I figured it was close to 100K so why not replace the whole thing. Since then the miles per gallon went downhill , really bad. I was thinking about doing the basic (spark plug,wires, etc.) but I do not understand why the miles per gallon went down. Also nothing else went wrong with the van, like dash lights or anything else. Anyway thanks for replying.