Engine Performance problem
2007 Buick La Crosse 6 cyl Front Wheel Drive Automatic 14232 miles

I get 15city and 23 highway shood be 20/30 dealership had the car several times they connected it to computer said nothing wrong with it tried everything tire inflation,clean air filter fuel injection cleaner took highway trip used cruise control 95% of time checked mileage 23mpg dealership said bad driving habits but I turned in a 97 LSS which was getting 19/30 is there anything you can suggest thanks Vince from Pittsburgh