Instant MPG readings aren't the best way to measure consumption at a given speed - they're notoriously fickle.
Better to drive a stretch of road in both directions, and reset/read the average consumption meter at the same points, at the same speed. Average the results.
I'm willing to bet you a donut your van gets its best economy just after the torque converter locks, like practically every other vehicle out there (
http://ecomodder.com/forum/showthrea...you-15182.html ). Anything beyond that is downhill.
Actually, in some vans and trucks, it's possible you may get best economy just after the top gear engages, but
before the lockup point, owing to lower road speed (lower aero drag).
It might save a little bit, if your A/C comes on automatically (as most do) when you turn on the defrost setting on your climate control.