Why would my Gas mileage be cut in half, Went from 400 miles on a tank of gas on 1997 Saturn SC1

Changed out fuel filter, Oxygen sensor, Crankshaft position sensor, to no avail, gas milage is still below normal. What next? Also deaccelerates when climbing hills. And acts like it has no power.

Asked by for the 1997 Saturn SC1
sounds like you have a clogged cat converter.

But it's not getting hot.
does not have to get hot for the cat to be clogged. have a shop do a backpressure check and a snap throttle check to verify