Ignition timeing vs MPG.
 

I discovered that the previous owner set the ignition timing on my prizm without the diagnostic jumper connected, end result is that the car was at 4 degrees BTDC with the jumper connected when it should be at 10. Now it idles at the 16 degrees BTDC vs 10 it was. What kind of FE gains should I see?

Advancing the ignition "too much" according to the OEM can increase power, increase fuel economy, make starting easier and increase NOx.
So do some ABA testing, you may find the further advanced ignition is better for FE.
Giving an otto about 8 more degrees of advance over OEM recommendations is pretty common.