First, if your gun looks anything like
you should immediately toss it in the nearest garbage can. It doesn't matter what brand it is, what specs it claims to have, etc - they're sold under at least a thousand different names, and none of them are worth owning.
How large of truck do you have? A 1/2" drive impact wrench will work great for anything up to a 1-ton pickup, but for larger you'll probably want a 1" gun.
Brand matters - if possible, get Ingersoll-Rand, Mac, or Matco. Some people like Snap-On too.
The best bang-for-your-buck seems to be the IR231 series:
You can pick up a new one for about $125-$150 depending on where you shop. If you want to save money, you can pick up exact clones of it under other brands for $40ish, that seem to work almost as well.
If you have a big truck, get a 1" drive impact wrench. The harbor freight $119 one works quite well.
For a 1/2" impact wrench, you'll want to use 3/8" rubber air hose. Do NOT use 1/4" hose, plastic coily hoses, etc. For a 1" impact wrench, you'll want at least 1/2" hose.
If your air compressor is crap, the regulator might not be able to supply enough air - the classic symptom is the tool running at full speed for a fraction of a second then slowing down. For an impact wrench on a 125PSI or less compressor, you'll want the regulator cranked as high as it can go, not set to 90 at no flow - the tool's 90psi recommendation is _at the tool_ with the tool running, so the regulator has to be set high enough to compensate for line losses, etc. If you have a really poor regulator, either replace it with a better one or plumb a second air coupler directly to the tank for using your air tools.
Use good couplers - import ones will work, but good 6-ball ones from your local supplier of quality air tools will work much better. Cheaper ones often restrict the air flow a lot.
--Bushytails