Ok so back when i lost a bunch of weight the first time around i never joined a gym or owned any equipment i just ran outside and did some resistance strengthening exercises, the second time i lost weight i did the same but experienced real bad shin splints at first b/c of overtraining, and had to wait it out about a month to start back on my running routine, well now im loosing weight again and i got my shin splints, again, and i decided to join a gym so i can hit the elliptical machine and still get my cardio while my shin splints heal up enough to take it outside....well i moved so now i don't have a good place to run outside im wondering if i can substitute the treadmill at the gym im at instead...the thing is i was in the Navy's DEP program basically meaning i was signed up but waiting to ship to boot camp...i didn't meet weight requirements when it was my time to ship so i was discharged, so now im dropping the weight again to get back in the Navy. Now obviously i know the treadmill is going to help me loose weight, regardless its cardio and im going to burn calories but my question is:

Will running on the treadmill instead of running outside affect performance?

i question that b/c running outside its all you moving and changing pace as you wish, with the treadmill the ground is moving and your just keeping up, our recruiter always used to say when we were about to do our runing PT: "ya'll better get ready this aint no treadmill"....so i dunno, anyone know or know from experience the difference in training on both?

the biggest difference, I've found, between outside vs inside on a treadmill, is you don't have wind resistance on a treadmill - so you're p robably working harder outside.. plus it's way less boring

There is a few things I would like to add that I learned on my own. Doing your workouts in a gym on a treadmill is way more safe! I have a scar on my leg from a dog bite to prove it. A lot of gyms use treadmills that are low impact and much better for your joints than the ground. You will want to walk on an incline so that it will better similate walking on the ground. On a treadmill the ground moves and you just walk your not really pushing your own weight. So what you need to do is have the incline around 2% I like to use 3%.

Wind resistance is one factor. Another factor, I've found, is like you first stated:the ground on the treadmill moves. Alot of people can run say a mile easily on a treadmill, but when they hit the pavement they can only go maybe 1/3 of a mile and break down. I remember when I first started out, I couldn't run outdoors more than 3 tenths of a mile to save my life, but if I did a treadmill run I could pump out a 1.5 mile in under 10 minutes

Personally, I prefer outside over a treadmill. The only times I generally run on the treadmill is when the weather denies an oppertunity for outdoor running. If the weather is bad and I have to go on the treadmill, I usually do 1.5 times whatever I would normally run outdoors. Say I usually run 2 miles outdoors, I would multiply that by 1.5 and get 3 miles. I would then do 3 miles on the treadmill instead of 2 outdoors.

I feel running outside gives you a much better workout. So IMO if the weather permits it - run outside. If not, a treadmill is fine.

running outside is fun, you can go places, see people and such, but also once you start to tired you will slow yourself down, but on a tredmill you cant slow yourself down or you'll face plant, so i find Tredmills work you more, and hurt your joints less, and if you are in a gym, if you have an accident on the tredmill, like roll your ankle bad, or if you work too hard and you pass out, (happened to my dad) , there are people around that will help you,

they each have their pros and cons, but i believe that it wont affect you much either way

I think they each have pros and cons: Like merciless said above, most people find running outside a lot harder than on a treadmill. So if your goal is to run a race or play a sport without getting tired, you can't train just on a treadmill, unless you're willing to go faster on a higher incline in order to be able to go slower and on the flat outside (work 2x as hard to get half the results in terms of cardio). There are pros to a treadmill, I have bad knees and when I run on the side of the road the slight slope always throws my weight distribution off a little, so it aggrivates everything. A treadmill is nice and flat, no bumps or uneven ground to trip on or stress one side of the body more than the other.

If you're looking to run for the fitness of it, weight loss only and some cardio improvement, then a treadmill is probably ok. But if you're training for an outdoor sport or are wanting to do any running outside, say for a 10k or triathlon, you'll have to do some running outside.