I don't know if it has been touched on or not but how long (I know it's a relative question) will a PowerTank last (say 15lb.) when airing all fours up from 20 psi? How many times can I do this? Where do you fill it? What's it cost to fill it? How much of the gas would I use if I chose to carry an impact wrench (I think that's right) to quickly remove lug nuts (sounds stupid now that I asked). Longevity and the cost of gas would be my main concern. Air is free, from a compressor stand-point. Basically, what is the cost to benefit besides portability?