Outdoor - Completely flat, little wind, similar surface (let's say a rubber track)
Indoor - Treadmill flat, same temp as outside, mph/kph calibration is correct (this is hard to prove or disprove but if you are an experienced runner you know if it is wrong)
NOTE** Just because a treadmill is on a flat floor doesn't mean you are running on level. Also - and more common - just because a treadmill is on a LEVEL floor doesn't mean the "0" level is actually level. I have seen both inclined and declined slopes for this. I have worked out at a gym where the owner kept the desirable (good location/tv) treadmills inclined on "0" so the heavy runners wouldn't use them.
Then they are the same. I have worked with people that have had faster treadmill times and others that have had faster outdoor times. It is really in their head. Personally I have the attitude that I will do anything I am "made" to do so I think I am faster on the treadmill - yes it is in my head too. Others get sick on treadmills and need to run outside. The question is very very similar to saying do you run faster on a track or road running? People have the same variances.
The 1-2% incline comment. This comes from road racers. From the road racing/marathon community they believe setting the incline to 1-2% should mimic times they would get on average when out on the road. In theory it is probably right on average but a better tool would be going between -5 to 5% to mimic the hills you will face - I know that is only possible with very high end treadmills.
Just like the outdoor winds and hills and rough road conditions you may face you can also face indoor conditions. I have ran in small rooms where I could tell the oxygen levels had depleted during my run, I personally can't stand running in humid rooms, I run slower with a video screen in front of me... We all have our likes and dislikes. If I had my way I would be in a room at 50F with little humidity at 1 PM.
Unless your goal is marathons or longer road racing you do not need to make an adjustment on your treadmill. If your treadmill times are vastly different from outdoor times I would try a few other treadmills to make sure the calibration isn't throwing you off. If the times are still different then you need to see if there is something you can do to help your "slow area". If your outdoor times are slower are you sure the running area is flat, are you sure about the distances, do you like where you run, are there obstacles, do you run at the same time of day compared to indoor, how is the weather???
If you are really really concerned then count your strides on a treadmill for one mile/2km. Is it roughly the same outside for the same distance (and don't trust all outdoor signs that give you distance info). If your strides are the same that means that you are actually running faster on the treadmill (which is not a negative thing) and you need to push yourself outside more. If your strides are different you may need to work on lengthening your stride outside.