Which is the most suitable cycle in peugeot partner

What is the most suitable cycle in the use of Peugeot partner adventure 2.0 hdi, usually
I do not exceed 90 on the intercity road, while the 4th gear on the road goes at a speed of 90 km, the speed is 2500, if I shift to 5th, the speed is 2000, which is more suitable for the engine and for burning less diesel.

It is always better for upper gear and low rpm fuel consumption.