I do more than 80 on the highway and get around ~25. Stock tune, 93 octane gas from Shell or Sunoco.

-Any codes?
-Replace your spark plugs and make sure to gap them correctly. These cars eat them quickly.
-Make sure your air filter is clean. You could try changing your fuel filter also.
-Clean your MAF with MAF cleaning spray.
-Run Sea Foam through the top end.
-Clean out your vacuum booster/suction pump (look it up on ECSTuning to see what that looks like) - you can use brake cleaner, carb cleaner, degreaser, Sea Foam, etc.
-How's your engine temperature? If your thermostat is bad and the engine runs cool, it'll kill efficiency, and a bad coolant temperature sensor will make it just run poorly.
-Your O2 sensor could be kaput even if you're not getting any codes. They wear out over time. You could try changing it.
-Check your boost with a gauge and/or do a boost leak test.

 

I just did the test: full tank, 230 miles. around 15.3mpg.mixed, that is too bad

The tire pressure is ok. 3.5 P.

Will check O2 sensors and spark plugs and other stuff.

this car is a free gift from one of best friend.
Yes, Audi is high maintenance car, just replaced the two front axle and belt tensioner. Seems the power steering rack has to be replaced too.

The car just pass 100 K miles, seems lot of parts need to be replaced...

 

will let my mechanic take a look and get the code.

Thanks a lot.!!