Experiment in mileage calibration

Whenever possible, I try to avoid the treadmill. Sometimes, due to a winter storm advisory -- or pure laziness, it cannot be helped.

I've always suspected that my treadmill wasn't totally accurate and resolved myself to the notion that I just think that I always seem to run faster outside because running inside seems to take the wind out of your sails. But I felt like I was SO much faster outside.

This morning during my run, I decided to use different music, which happened to on the iTouch. So I lauched the NIKE + GPS app and played the music through the app while running.

After a few minutes, I realized I was still holding the iTouch and discovered that the NIKE + GPS app told me I hit 1mile. When I looked down at the treadmill I saw it was about 0.1 miles behind.

Hmm. That's strange I thought. Maybe I didn't start at the same time.

Nope. That wasn't it. I had the time synced up as well, so that it was running at the same time.

Ok, well after mile 2 the NIKE + GPS was now 0.25 mile ahead of the treadmill.
Now I'm thinking, the treadmill is a old ProForm model. Probably older than my daughter. So maybe it's a little bit off, no big deal.

Now, I notice the differential is almost 0.5 mile! The NIKE app says "4.25 miles" while the treadmill says "3.75 miles". So I start thinking that maybe all those 'longer' runs I had to put in on the treadmill of about 10 or so miles, was more like maybe "11 miles" or "12 or longer"?!

Which would you use?
     Which one is right?