Ok, I have actually heard some rumours that the 5th gear sensor that goes from the tranny to the ECU cuts power when in 5th for fuel economy????
What's that about? I'm aware of the sensor, but have no idea what it does, or why it's there. I don't notice a difference in fuel ratio, but I can't accelerate in 5th when trying to top out.. I usually get about 220 km's, at around 9000 rpm in 4th, a shift to 5th results in immediate slowdown.
Does this switch actually reduce power???? Timing is the only other thing I could think of, but how would that increase fuel economy?

 

Mixture is slightly leaned when in 5th gear and less than half throttle (as in freeway driving conditions).

Just putting it into 5th alone will not lean the mixture unless your TPS(s) is/are out of alignement.

 

ya in closed loop it runs leaner, you can see it on an A/F gauge. you can see it going back and forth more to the left. I doubt it makes any diff at WOT. sounds like you need a turbo