I use a Makita 3/8" to get lugs off with if I'm not using my air impact. I heard the big daddy Snap-On gun they just came out with has the most torque of all impacts. Your best bet is to go to consumerreports.com and check their reviews, they just did a test on impact guns a few months ago. That will get you the best bang for the buck.
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Automotive Electrical Specialist, Tuning Specialist