What does the path of least resistance look like for 10 gal per hour of air through a 3/8" line?  If we were talking honey or motor oil or something ... but AIR?
I don't think we can measure anything that small ... Jim S.

True Jim,  but don't forget you are dealing with positive pressure when flying so the whole system comes into play. One tanks pressure head may end up marginally higher. This could be caused by so many things that are small and hard to measure. Just an example, you could have a gas cap that leaks a bit more AIR on the side that drains slow.  So then the fast tank sees a bit higher positive pressure. Not being critical here but you did mention that one tank ALWAYS drains faster. Could be a couple of sharp edged fittings in the delivery line...no way to be sure.

Bill