Need help and suggestions about bad fuel economy

[BT91]

I have been getting bad economy over the past 6 to 8 months. I live in Canada just outside of Toronto. I have taken the car to the local dealership around 3 times for this problem as well as regular maintenance. Right now im averaging around 470 to 490 a tank until rapid flashing light appears. So 8.33 liters fro 100km which is terrible. When I first got the car I could get 620 to 650kms a tank no problems. The local dealer has found no obvious problems and say its the way I drive. I have not changed my driving style at all. The car is stock currently besides rims and tires for now. I have started driving like a granny shifting at 2000 - 2500 and coasting to lights and stop signs etc and fuel economy remains the same. I taking it to another dealer this week to have a second opinion. Im thinking fuel flush, new plugs, etc. Just thought i would get the opinion of some people on the forum before i dump unneeded money on the car. Your time would be much appreciated. New member to forum as well so any thread sugguestions would be great.