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??