[MetroMPG]

The main reason the mileage went down in '95 and up is because the body style changed, and the cars got significantly heavier.

I doubt there's any difference between the '94 ECU and the '95 ECU. In '96 the cars got OBDII, that's where the biggest difference would be seen.

Here's my advice:

Keep the '99 ECU specifically because it will permit you to use a ScanGauge in the vehicle (the SG requires an OBD2 computer). Using a ScanGauge to fine tune your brain and right foot will reduce your fuel consumption dramatically more than any ECU swap you could possibly make!