Assuming no technology changes (bad assumption), engine calibration is largely an exercise in balancing performance, fuel economy, and emissions. The rule of thumb is that in order to improve one of these characteristics, you will take a hit on at least one of the others. There are many exceptions to this rule, but for the casual tweaker, it is probably true more often than not.

1999 Contour SE Sport MTX75 T-Red/Grey
Audio: Pioneer DEH-7400MP (MP3 CD), Pioneer TS-A6857 front & rear, PPI PCX-250, Infinity Perfect 10.1
Other: cup holders, cargo net, Blizzaks, Potenza RE950s