The number of gallons that you need for any given tool is largely dependent on how often you intend to use it. If you are going to be running your impact wrench constantly, as you would in a professional garage, then you will need a larger compressor that can store more air and has a higher CFM rating. If you are only going to be working on one tire at a time on your personal vehicle, then you should be fine with pretty much any size of compressor.