FYI ....you shouldn't use a impact wrench on older vehicles. I'm not sure if you fit into this catagory but I thought I let you know. A torque wrench is preferable.
Many professional mechanics and tire stores use air-driven impact wrenches because they save time and physical effort. While okay for removing lug nuts on newer vehicles, impact wrenches really should not be used for reinstalling the nuts and especially not for the final torquing. If an impact wrench is used for installation, a torque wrench should be used for final tightening.
The torque applied by an impact wrench can vary from nut to nut. This can cause uneven loading of both the wheel and hub, and allow the lug nuts to work loose after miles of driving. Excessive torque applied by an impact wrench can also distort the roundness of the bolt holes, or even shear off the stud. It can also start a crack that will grow with time. If the nut should be accidentally cross-threaded, an impact wrench can cause stripped threads before you know it.