2002 Honda Odyssey poor gas mileage

I had a new transmission put in at 66000 miles and I am getting very poor mileage. 16 miles to the gallon around town. I used to get 19-20! What could be the problem? I tried inflating my tires more but that didn't help. I have had lots of problems with this car.We put a new air filter on and we change the oil at the right times.The Wiring burned up at 5000 miles. Gas tank gasket twisted and gas flooded the under parts of the flooring. 2 dead batteries. The warranty is over now so I would like to try and fix the rest myself. This is my first and last" new" car. I had the strutts replaced on both sides in the front as well.Thanks for your help. Sandy