KW PACCAR doesn't have any power when not on cruise

I just brought a T680 into our fleet and it's my first experience with a PACCAR engine. It is a 455hp with a 13 speed manual.

This may be a stupid question, I feel like many will know the answer, but Google hasn't been able to tell me. It doesn't seem to pull very well up an incline. I just had 14k in the box and you have to drop down to 10th at least up a smaller hill. However, when I put the cruise on, this thing comes alive. Set the cruise on 75 and she will head up the hill no problem.

Is this thing set someway that will limit power when not on the cruise. Some kind of fuel savings thing maybe? It was a small fleet truck, so maybe they played with the computer?

 

Yeah it's like it's a total different truck even. I've never felt it be so different before.