What incline at 2.8? That seems awfully slow to achieve a H/R of 130-140.
Notice I said 2.8MPH sweetheart!
I walk at 2.8 miles per hour, and the incline of the treadmill varies. I usually start flat, then progress to 2.0 for two minutes, then up to 5.0 for two minutes, then maybe back to 2.0 for another two minutes, and up to 12.0 for two minutes, then 8.0 for two minutes, 15.0 for two minutes etc....
If I feel I'm not working hard enough (I seem to be able to guess my heart rate fairly close, but still use the machine) I will just increase the incline. The longer I walk at a steeper incline, the higher my heart rate gets. By the end of a 15.0/2min @ 2.8mph, I'm in the 145-155bpm range. That's plenty high for me and the effect I'm after. This all depends on the machine too. Some machines will say I'm at X heart rate, and the next day I'm doing the same routine (with as many factors the same-as much as I can control) and I will be slower by 10-15bpm.
Whatever works, and this is what cuts me up like nothing else.
(By the way, I will divvy up the cardio once I hit 30min+ per session....15min treadmill, another 15min on The Gauntlet etc....)