My wife and I drove our A4 Avant to FL from PA last week.
First tankful was 25.8; second two were 30.5. (averages from the trip computer)
Nearly all the driving was on interstates, with a few stretches of two lane in the first tankful. We were actually showing 33mpg+ when running on some Georgia twolane at about 65; the remainder of the second two tanksful was in the 75-80 range. There was some more mountainous running on the first tank.
The first tankful was in PA; the second in Georgia and the thrid in Florida.
Would a difference in gas formulation have affected the poorer mileage in the first tank? Assuming a 'winter' formulation in PA vs. whatever is used in the warmer southern states?
Engine breakin? This was the first really sustained fast running on the car; it only has 4k on it after the trip?
Happy to hear any ideas.
Moon

 

elevation differences. if you start at sea level and your destination is 2000 ft, you will burn more gas going there than coming back.

also, ethanol blend gas gives you worse MPG. if they are blending with ethanol in the north, that could also contribute to the lower MPG. ethanol is used to "winterize" gasoline. i don't use ethanol blends. i don't trust it with the valve seals.

 

That echoes some of my thoughts. We were starting at about 2k, and our run south took us to nearly 4k in West Virginia. Now, that said, we didn't seem to get back much mileage on the downslope as we drifted down to the Piedmont in the Carolinas. All that was on the first tankful.
The ethanol (or other winter blend) crossed my mind as the main culprit. It was early fall when we picked up the car, and a maiden run on the PA turnpike netted 30+ with favorable wind and grade, and 28 coming home against both. We haven't had much trip mileage since then (been one hell of a winter in PA) plus the Blizzaks surely have more rolling resistance than the all seasons. For our trip, I put the OEM back on.
I avoid ethanol when possible, but the last tank of the trip had a 10% ethanol posting, and our mileage came back to our rock-steady 30+ as we buzzed on down I-75.
In any case, mighty pleased with the mileage. We had an AWD 2.8 Passat wagon previously (an earlier A4 mechanically) which got 23 mpg on the same run last year, and it didn't have nearly the power ( or maybe torque is more the issue) of our new car.
Moon