Economical gear change speed/rev's

The manual for my NJ Pajero (under Section 6-8) has a table that shows recommended speeds to change each gear. It states that "The shift points shown are recommended to assist in maximising fuel economy."

The table recommends (for the 3.0L model) gear changes as follows:

1->2 - 25 kmh
2->3 - 50 kmh
3->4 - 80 kmh
4->5 - 115 kmh

Sorry if I seem a simpleton, but my simple logic tells me to change when the rev's are as low as possible but high enough to cater for the new gear at the given speed. In other words, keep the rev's as low as practicable.

If I followed the table in the manual diligently I would never get out of 3rd gear around town!

From answers to my previous posting about using 5th gear around town it seems it may be to do with when the peak torque is delivered. I don't have a fantastic grasp on this, so can someone please help me to understand this better?

Thanks!