FYI:
Fuel Economy of 1990 Plymouth Voyager/Grand Voyager 2WD
I think
this is your Voyager's EPA rating: 19 mpg (combined) 17 city / 21 hwy. You can do better! (I assumed the premium fuel requirement for the 2.5L engine means: turbo.)
(Too bad it's automatic instead of 5-speed.)
One of the first recommendations is to get a fuel consumption gauge. Instant feedback & resettable average consumption is critical to playing the MPG game. Since your van is pre-OBD-II, you'll need to look at the MPGuino. (Safe to assume you've got some mechanical skills? You'll need to locate/splice into a few wires.)