[Big Dave]

Aero mods are always effective. there have been a number of folks on this forum working on van aero mods. My suggestion: You now see the big rigs with four-petal tail fairings. You could make something like these. Their fairings fold back when they go to the loading dock. You could manually fold them back for city driving.

If you are really serious about improving MPG, you simply have to slow the engine down for a given road speed. Your Astro probably has 3.55 gears. If you can retrofit 2.73 gears that would slow down your engine by 24%. That will result in a 55% reduction in engine frictional HP load. EFHP goes up with the cube of RPM. With my exotic drive train I have achieved a 34% reduction in engine RPM and a 70% reduction in EFHP.

Caution: Lower engine speeds are murder on automatic transmissions. If you go to the effort to re-gear the van, find a reputable transmission shop that rebuilds automatics for hot rods. Tell the guy you intend to run at 25% less RPM across the board all the time.

Most automatics use the torque converter to pump ATF to the cooler, but by slowing down the engine you reduce the ATF flow to the cooler and heat builds up. Good hot rod oriented shops can provide boost pumps to restore the ATF flow to the cooler.

Not an issue to me as I have a stick, but I've never seen an Astro with a stick.