Originally Posted by BigD
Somebody set up the bomb to your calculator. The EPA rating on your car is 17 city 21 highway, and EPA ratings are absolute best case conditions - done on a dyno, at constant speed and load, accelerating like grandma. In other words you'll never hit those numbers, reality will be much worse, especially if you drive with any kind of enthusiasm.
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.