Hi stonking,
My idtec Tourer seems to tell me to change up at 2000 revs, and to change down at 1500 revs.
1500 revs equates to 40 mph in 5th gear, and 50 mph in 6th gear on the speedo.
in the 50mph (average speed camera/roadworks) I run at about 52 on the speedo (which I think is a genuine 50) and this just keeps me in the rev range for 6th gear, without the green warning light to change down.
If you keep within these limits the MPG is good.