Artemis,
I have a 91 Plymouth that is OBD I. I only use an EFIE and am getting great results -- about 45% increase in MPG for city driving. From what I understand, working with OBD I iseasier than the later OBD II, and you can be more agressive in your EFIE settings without throwing a check engine light. This, couples with being able to manually adjust your spark timing, may allow greater gains then are possible with the later systems.
Steve
1991 Plymouth Acclaim 3L V6.
1 dry cells with nineteen 6"x8" 316L ss plates, driven by constant current PWM set at 35 amps (13.3V at PWM). 28% KOH electrolyte. Total measured output 2.5 lpm. Mileage went from 18 to 26 mpg, all city driving (44% increase). EFIE set at .370 and I still need to play with ignition timing.