Super low MPG in 91 Tercel

You are chasing the wrong dog here. Check cam timming and engine timimg. As I said earier my son has a 90 and it is very sensitive to ignition timing on MPG.

Cam and ignition timing were my first suggestions, many posts ago. The OP says that they are both OK.

If timing is stock, MPG will not be good on that year engine in a tercel. You need to run about 6 to 8 degrees before TDC and run at least 89 octane or better in warmer weather too. Leave it stock with