Originally Posted by chromius
It's absolutly not "the best case scenario" If you look at how they do their testing.
Their highway testing is done with a 16km 'simulated' run at varying speeds between 77 km/h and 97 km/h (not a constant speed). In the 540i 6th gear isn't even really useful until about 110km/h, If I use 6th at 97km/h I don't get great highway millage because the engine is lugging (as much as a v8 can). Personally I get my best gas mileage at 120+km/h in 6th with cruise control on, and I see an average of 8.5L/100km on the highway, sometimes even better.
The gearing on the 540i is the key factor in this situation, that is why you will consistently see 540i6 drivers getting better highway millage than the EPA estimates, and it's also why the EPA estimates list the 5 speed Auto 540i as getting better hwy mileage than the 6 speed. It's simply because their testing is not suited well to the 6 speeds long gearing.
Cool, I have been misinformed (edit: what load do they use though, that is the key. You can't measure how much you car will have to work in the real world on a dyno, when air resistance builds up exponentially, and not all roads are flat, plus I bet they accelerate slowly - well used to, that's why the 'adjusted' rates are much lower). Do you actually figure your consumption with real numbers?
How do you account for independent tests though, who consistently get worse mileage than EPA and as I noted above, the 540i6 is no exception, with MT averaging 18.