I have lots of opinions about treadmills. First, I run slower on the mill than I do on the road. Most people think a treadmill is "easier" because all you do is lift your legs and the "road" passes underneath. I disagree. First, I know I run slower on the mill. I can compare heart rates and paces and I can runner faster on the road than on the mill. Second, as I have read, your body gets no forward momentum from the mill. Think about running a 7:00 mile and trying to stop suddenly. That is the benefit of the forward motion. I am no physicist, but the forward motion has to create momentum, which is not present on the mill. I suppose the effect of these two forces (loss of momentum vs. reduced effort to create the momentum) vary based on running style, form and fitness, but for me I run slower on the mill.
And please, please do not set the incline on the treadmill to 2% to compensate for not running outside. First, there is nothing to compensate for as far as I am concerned. Second, as I have been told, it increases the chance of knee injury. On an incline, your foot and heel don't land flat. In fact, they land at an angle, and thus prolonged running at an angle produces unusual stresses on feet heels, hips, backs etc. I have had several doctors and therapists tell me to never, ever run on a treadmill with an incline. If you want to increase the workout from a treadmill, run faster, not at an incline.
I am curious if others have similar experiences. I know there are threads about this on Runners'' World, and no definitive answer, which I find shocking, really.