Purchased 2004 A6 2.7 s-line with 124,500 miles. Gas mileage HORRENDOUS! Visited Plaza Audi here in St Louis; basically tuned the car, $725.00. Still poor gas mileage. Visited Audi repair, AIT flow meter replaced; $636.00. Gas mileage still horrible. Can anyone help?

 

What do you mean by horrible? 8 mpg, 15 mpg, ?????????? Any fault codes? What are the fuel trim values? With 124,000 miles it MAY be that the O2 sensors are just plain worn out and not reading right. Just because there isn't any fault codes for O2's, means nothing.
Check also to be sure there's no excessive brake drag and that the odometer is accurate. Do not rely on the "AVG MPG" shown in the display, they are not necessarily correct. They infer mpg using a computer model, there is no actual fuel flow sensor. Fuel used is estimated by using a number of parameters.