Actually I was wrong too. They list the older, small, rear-tank as 37 gallons. In 2010 they changed the spec to 40 gallons, which I think correspond to the change to plastic?
I started thinking about it because I sized my underfloor storage to fit the old 37 gallon tank. I'm not sure the nominal empty measurements actually changed though. Maybe the switch to plastic lets it bulge enough to fit those few extra gallons?
2000 E450 dually V10 wagon