IMHO  there is little difference if any in fuel economy if the cat-con is
absent or present in good shape. The O2 sensor is located before the cat-con,
not after it, and therefore would read the same, cat or no.
   The only difference would be slightly less backpressure with the cat
deleted, a non issue if stock mufflers are retained.
   Note that none of this applies to OBD2 cars, (1996-up) which have o2
sensors before and after the cat and much more programming to monitor cat-con
efficiency. They will still run the same,but will set off the check-engine
light.