Hmm.. I thought so..

Q: Is treadmill running easier than running at the same speed outdoors?

Short answer: Yes.
Longer answer: Treadmill running may feel more taxing, but physiologically it's actually a bit easier than running outdoors.
In his book Treadmill Training for Runners, Rick Morris explains, "Running on the treadmill, you don't have to overcome the effects of wind resistance and you also have that assistance of a moving belt doing part of the work for you." 
To more closely simulate road running, set your treadmill's incline at one or two percent.

This I just found on the Active.com site, by Alissa Hardiman from Runners' World.

I would also suggest have a fan blowing on you.

A BIG fan.