...had to use my compressor to air up four 40's from 0 psig to 30 psig. I ran the compressor non-stop for about 30 minutes and had them all aired up and ready to roll....
That's pretty impressive. What was the weather like at the time? I only ask because I had mine out during a trail run in August in the sweltering heat (direct sun, mid afternoon, temperatures over 100°), and after airing my stock 265/70/16 tires from 20 to 32 psi, and another guy's 35's from 18 to 32 psi, it finally started cutting out repeatedly while airing up someone else's 33's.
After it shut off the first time, I picked it up and realized that the wiring was ridiculously hot, almost hot enough to burn my hand. I moved it into the shade, which helped a little, but it still took over an hour to get those last 4 tires aired up (compared to probably 30 or 40 minutes for the first 8 tires). All in all, still really impressive for a $60 compressor.