1STGTO,
Something is wrong. ECMs learn very fast on new vehicles ( less than 50 to 75 miles depending on how you drive) Even though mileage should improve as an engine breaks in, yours is not running correctly if you are driving as you say. Where in the country do you live? Mountains or cold?? If you haven't set a check engine light, you might still have a sensor problem. Have the dealer, one that knows what they are doing, put a TECH II on it and look at the outputs from the sensors with the vehicle running. Does coolant and Intake air temp look like it should?? Those are the two most common that could cause it to run rich. ( or maybe never go closed loop) Also have them look at the TPS throttle position sensor both closed throttle and WOT.
Ask the dealer..also when they have a TECH II hooked up ask if it's closed Loop...What do the O2 sensor read?? They will find something if you are really just driving normally at 12 to 16. In a 6 speed thats too low..
Here's another way we might help you on this board. AFter the engine is fully warm.Go drive 55 MPH on Flat level pavement. 1st in 5th gear at 55mpg, then 6Th gear at 55 MPH at both speeds with a speady throttle note the INST FUEL ECONOMY on that screen in the dash. Post it here. Others can drive their 6 speeds at the same speed and gear and quickly tell how far OFF your's is from normal MGP....Just a thought.
How about someone else checking thier INST MPG screen at 55 in 5th and 6th gear???and posting results.
I love a science project.....