An impact gun seems to tighten a nut as far as it will go therefore I guess this is over tightening.
If over tightening can cause wheel studs to damage/break then why do most mechanics use impact guns and why do we rarely see wheels coming off?
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Most mechanics will only use an impact to take them off. They'll use a socket/wrench to put them back on, then a torque wrench to tighten them to spec. It's someone at a tire shop (without guidelines) who will use an impact gun to put the lug nuts back on. A good tire shop will at least use torque sticks to ensure the lugs are not over tightened when they use an impact gun. That gets them in the ballpark and then they torque it down the rest of the way.