In my experience, what differentiates between a "dragging" tune and a "driving" one is not the tempo, but WHERE you sing relative to the beat. If you sing slightly ahead of the beat (as in most bluegrass songs), the song has a "driving" feel. If you sing slightly behind the beat (as in most jazz), the song has a "laid back" feel. Note that this is an entirely different issue than playing fast or slow, or speeding up vs. slowing down. If you have a drum machine available, the easiest way to see this effect is to have a rock steady bass drum and then adjust the snare or high-hat so that it is either slightly before or slightly after where it "should" be. It often takes a lot of practice to learn how to adjust your singing so that you can sing ahead of, on top of, or behind the beat. But the effect is worth it.