I think there must be something I don't understand about impact wrenches. After snapping a half-inch breaker bar (with a cheater) while trying to remove the axle nut, I decided that an impact wrench might be a good investment. Between positive reviews, a $99.99 price, and a 20% off coupon the Harbor Freight Earthquake seemed like a good bet.
Hooked up the wrench, set the compressor to 90 psi, grabbed a 30 mm impact socket and went after the axle nut. Sure enough, it came off. Good. Switched to a 17 mm socket and started in on the wheel lugs. It wouldn't touch them, not even close. The wheel lugs are torqued to 120 NM (90 ft/lbs) and came free easily with a 18" breaker bar (a 25" bar wouldn't touch the axle nut). I let the compressor come up to full pressure, I can't think of anything different between the two fasteners except for the size (and that one was initially much tighter).
Can anybody explain this? Is there a lesson about impact wrenches in this? I'm stumped.