My truck change its gear at higher RPM (15-18 sometimes 20) what would be the reason for this?

Hello, The newer trucks use intelligent shifting to determine best drivability and fuel economy. Especially with a heavy load, however, I would expect the shift points to be close together. That makes me wonder how the transmission is set. Especially those that are made for the euro markets, they often have several settings for mountain, snow, etc.. kinda like a land rover or jeep. The shift points will change with each setting, and so will the amount you can pull efficiently. If you are dead-heading with those shift points and the transmission is set to normal drive, take it in and find out what you've got going on. You'll make the cost back in fuel it sounds like. If this doesn't answer your question, or you want more discussion on it, give me the specs for your truck (which engine, 12 speed trans?, which shifting tech you have).