HI thee...

This is my first post and, I had not enough time now to read much on the forum. I hope I would not re-ask question previously answered.
I do know some tricks to have a mile or two on my full tank. Fortunately here(kuwait) I pay 0.9 USD for a gallon of gas and I would only think of how to get 100 HP more for my Red beast. But, I'm planning to have a long trip over the summer and to drive it over to Europe.
Are there any more alternatives beside an light foot on the gas pedal and properly inflated road tires?
I ask more of a way to remap the ECU and even too loose some HP in the idea of gaining some more MPG?
Woud the dealership have such solution?(35 k km on and still under warranty)
Thank u for your help.

P.S.
Has any of you had a long trip with the Xterra? I never drove more than 200-300 km in one day. How dose the car takes 2-3 hours of driving?