At 1.5-3% incline, you've more than compensated for the lack of wind resistance on a treadmill. So, yes, you should definitely count it. I ran the vast majority of my miles on the treadmill last winter because I had 2 falls on the ice in one week. I kept it at a 1% incline. I still did my long run out on the road because I could run that on the weekend and at least have the advantage of daylight. I have to say, now that I'm predominantly back on the road, I've been surprised that there has been no issue whatsoever with the transition. I thought that there might be some soreness or whatever since my legs weren't conditioned to the road, but that didn't happen. My times and fitness have definitely improved, so I am a firm believer in the treadmill!