I am thinking about this a lot lately. Most of the runners I've ever gotten have been too small, but I've noticed that my favorites were the ones that I got in running shops, where the staff are literally runners, and know about running. For simplicity, lets say your perfect fit will give you a one thousand second 5k. What would the difference be if your runners are a half, full size too big/small etc. Would anyone have an idea of an estimate for this scenario? Keep all other factors equal. I'm annoyed that I have gotten some crap recently, by looking for cheaper deals in the shop. I finally went to a proper running shop, and got a gait analysis, and fitted with the right runners. I haven't done this in 3 years. I've been randomly getting different runners, but I have a habit of getting them too small. It is more expensive to go to a proper shop for your runners, but worth it in the long run I guess. I'm looking forward to seeing how my next race is in comparison to the crap I've been running these days. Thanks.