In response to the OP, I experience exactly the same with my 2.0 JTDM SW. The gearing is very tall and at 40 mph it has to be 4th gear. At 70 mph I'm looking at a little under 2000 rpm in sixth and is much more economical in fifth. I decided to stick to sixth on a run yesterday driving a little over 70 mph and mpg suffered ~40 mpg. In fifth at the same speed it would definitely be better. I've come to the conclusion that sixth is for driving 80 mph+ for optimum mpg on real roads without continuous shifting.