Air impact wrench sizing?

HI I have a '00 K2500 Suburban diesel and want to get an impact wrench to help when rotating the tires. I know the proper torque is 140 ft-lbs. What
capacity air impact wrench will do the job? I thought 350-400 ft-lbs would be OK, but my BIL (similar truck) says it's not enough - should be at least 500 ft-lbs. What's up? Thanks
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Ooops - make that a '99 Suburban.

impact wrench to help

would
least
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my Chicago Pneumatic CP-734 is only rated at 325 ft lbs and has never failed to remove any bolt/nut I have asked it to. the key is a proper air supply (3/8 ID hose and fittings) and a quality tool. the central pneumatic stuff the harbour freight sells is just junk, wouldn't even touch my lug nuts.
hth, Bret
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Thanks for the reply. My BIL has offered to let me try both his medium duty and heavy duty wrenches, so I guess I'll bring along my air compressor to see if it makes any difference. Walt

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