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I've found that by running comfortably on a treadmill for 30 minutes, I average 9 km/h, and I've tested this on several different treadmills.
When I go running outside, RunKeeper or some other tracking device tell me my pace is closer to 11 km/h , 12 km/h.
I'm wondering what the likely reason for this? Perceived difficulty is different between the two, device(s) are not accurate, having to propel yourself forwards means you naturally go faster??
Treadmill running is much easier, unless it it one of the running road, and would be the reason for the difference in pace. The reason I say this, is because the treadmill does all of the work. All you are doing is keeping up with it.
The other reason, is outside the gradient naturally changes even under foot. Your body has to work more to move in a forward direction than on a treadmill. (ouside is like working on an unstable base in the gym - its harder) you also have the weather to contend with. Even a flat route outside would greatly use more muscles and effort.
Personally, I would only use a treadmill to warm up, or for a run when the outside option was not available.
Given that outside running utilizes different terrains, some steeper than others, it's possible that your terrain slopes downhill or you usually run with the wind, thus making your speed faster.
It could also be that the mechanics by which each device measures speed varies slightly (due to the materials used).
Lastly, given that on a treadmill, you are actually exerting more speed to counteract the backward movement of the belt (running outdoors doesn't have that), it's possible that your net speed is reduced, thus the values shown on the treadmills.
