I don't know. It shouldn't be that large a difference, but there it is. My strategy now is to run it low, but not quite to empty.
I did notice after filling up this morning, my glide points were off - rolling farther from the same starting speed. I do LOTS of P&G, so maybe the weight difference is more relevant there.
I like your idea of siphoning out gallons and then adding them back later.
