1/2 inch impact

Depending on what you're trying to do you might want a stubby impact. I picked up an AIRCAT stubby impact (linked below) and Have found it extremely usefully for things like taking stubborn brake calipers off or exhaust bolts. Now, while it can and will take tires off I wouldn't use it for doing that job on a daily basis. For my day to day tire changing and really stubborn bolts (where it can fit) I use an Ingersoll Rand.

AIRCAT
http://amzn.to/1lCrH0Y

IR
http://amzn.to/1lCrU4g