If you took your 6 hp compressor and hooked it up to your truck engine, you think it would work the same? Of course it would, because the truck engine has MORE than 6hp, so it would run the compressor.

The air brake compressor isn't made for filling tires, so it only puts out so much, AND you don't want the thing robbing tons of HP from your engine(sorta like switching on the A/C in car) so they limit the size and output of the compressor. The air brake compressor will fill a tire no sweat, I use it all the time on my semi.

Your home compressor is running on 110AC, a lot more power available than 12DC.

What I'm saying is that if he wants to run big stuff, he's better off with a PTO/Belt driven pump.