My son bought a stock 92 Justy with 5spd, 4wd and MPFI.  We put a new Cat on it and it seems to run fine. We are only getting 31mpg at 55-60mph.  We though it should get more in the range of 38-40mpg.  Any ideas?

Air/fuel feedback system: O2 Sensor, IAC valve, MAF sensor.  MAF sensor and IAC can be cleaned. 

 

Valve lash adjustment.  These engines have manual valve lash adjustment.  Super easy on the Justy.

 

Check plugs, clean, gap.

 

Also check for weak spark.