We found the effect of wind resistance to be an increase of about 7%, when comparing treadmill to over-ground running at 6:00 ad 5:00 paces, which I believe corresponds to about 2% grade using the 12 sec per 1% grade factor. Some suggest putting 1% on a TM to correspond to overground cost; I have usually used 1-1/2 or 2%, the latter making it a safer conversion. When I was coaching Ken Martin he did all his (weekly) 20-mile runs at 6:00 pace on his TM at 2% grade and was at 7000 feet of altitude. That sure must have made running just over 2 hours in NY that year (sub-2:10) seem a lot more pleasant -- at least the scenery was constatnly changing