Buying an air impact wrench is an important addition to any do-it-yourselfer's home toolkit, but there are also a couple things that you need to remember before you go out and buy one. These are the size of your air compressor, what size drive you need, and not under-estimating how much power you'll need.

The size of your air compressor is probably the most important thing you need to consider before you buy a brand new impact wrench. The air compressor is the power source, and if there isn't enough air pressure, even the most powerful and expensive wrench on the market will not work up to its potential. To make sure your air compressor will work, you need to know the "air consumption" of the wrench (this will be measured in CFM, or cubic feet per minute). So long as your compressor can provide air at 90 psi (the minimum pressure required for proper use of your impact wrench) faster than the wrench will consume that air, your air compressor will be good. Any less, and you very quickly lose torque as your impact wrench uses up air pressure faster than your compressor supplies it.

Another important thing to determine is what size drive your impact wrench should be. A lot of people immediately think that bigger is better, but that's not necessarily the case. You really don't need a 1" impact wrench to loosen a 10mm bolt! And you definitely don't want to try to loosen a 2" bolt with a 3/8" wrench. The best thing to do is to figure out what sized nuts & bolts you'll be using your impact wrench on most frequently. Then find the impact sockets in that size, see what size drive they're available in, and get the drive size that will work best with the size of sockets you'll be using.

You can get adapters to fit different size drives with sockets, but at the very least, it'll give you a quick and easy way to narrow down your search. And why bother getting a different size than you need?

Finally, a common mistake is to under-estimate how much power you'll need. Taking off the wheels on your car is a common example. The manufacturers recommended torque for your lug nuts is probably between 80-140 ft-lbs, so people will "play it safe" and get a wrench with a 220 ft-lb max torque. The problem with this is firstly that the max torque in an impact wrench is always more than your "working torque", and not all companies give out that information.

The other problem is that you are not taking into account any real life factors, like rust and other weathering, that will make your lug nuts harder to get off. For most cars and light trucks I recommend a wrench with a max torque of 4-500 pounds, just to make sure you've got enough (it would suck to have to manually break each of your lug nuts because your new impact wrench isn't powerful enough).

For uses other than taking off lug nuts, you'll want to find out what you're suggested torque rating is, and then add a decent safety factor to account for the difference between max torque and working torque (if working torque isn't provided by the company), and for any outside factors that'll make your nuts harder to get off.

Whenever you're buying a new impact wrench, you want to make sure that you remember to take into account your air compressor, the drive size, and how much power you will actually need.