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One thing I found out on my Astro is that the gearing makes a lot less difference on a van than it does on a lot of other cars. I get better mileage at 70 than I do at 45mph. Also driving in 3rd gear drops me at most 1mpg but typically is just like 0.3mpg worse than 4th gear, so changing to a 4 speed or rear end gears might make the engine last longer if you don't haul heavy loads, but probably won't change the mileage enough to even be able to measure it.
A few things I have found so far that help is to get rid of the belt driven fan and set the ignition timing at the proper setting, advancing it won't really help anything but if it is retarded any at all it kills your mileage. I am next going to try a tbi spacer and advance the cam timing to increase bottom end power for towing and hopefully pick up some mileage. While I have the tbi off I will probably do the 'ultimate tbi mods' to it so it improves throttle response.
The biggest gains on a van seems to be aero modifications. The underside of a van is terrible so adding some panels to smooth the airflow will be worthwhile and otherwise be invisible. Doing something to improve the wake coming off the rear would be a huge gain in mileage but I am really not sure how to go about that without making the back doors useless.
I am just starting the experimenting with my Astro so there is a lot I haven't figured out yet. The best thing to do first is build a MPGuino and start actually getting some data about your mileage so you can tell what is working.
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