hello. does anyone have suggestions on how to get better gas milage out of a 1999 crown victoria?

I had a 1999 Crown Vic that got 25-27 MPG on highway. I changed the stock single exhaust to duals with glass packs. Performance & fuel milage jumped to 25-27 on highway. Before chabge was getting 18-22. Worth a try. Don't know what rear it had under it.