^^^^^
An excellent point! Endlessly discussing the capacity of the Passat's tank in terms of the miles that can be extracted from it is meaningless absent all the other variables. Some other recent thread has posted some simple, highly revealing graphs of speed v. mileage, and for several vehicles. It doesn't much matter, given the fact that air molecules start to "stack up," i.e. make for more wind resistance about 40 mph, and the progression is not linear with increasing speed, but becomes exponentially greater*. Above 65 mph, the air dictates a very noticeable law of diminishing returns: one pours in a disproportionately larger amount of fuel for each mile per hour increase in speed. This is why I'm not concerned with my mileage, beyond a certain point. We travel the posted limit in town, 10 over on the 2 lane state roads, and 75 mph on the Interstate...because that's how we drive, and the mileage is what it is at those speeds.
*"I use the word exponentially cause I learned it in a math class, and that's the first sentence I've ever f****** been able to use it in." - Lewis Black