670... that is correct. I too see a 8-10mpg hit with all three of my boxes on. A little less on the slower state highways and slightly more on the Interstates...Oouch!
I found these...
Giant Loop Gas Bag Fuel Safe Bladder | eBay
They have 1gl or 2gl available for the same price... :{)
When I looked, 1 gallon size was $35 more than 2 gallon size.