Gas Consumption on 98 Honda Accord

Check engine light goes on / off on my Accord. Computer says that Catalytic cconverter is not working to its capacity. Does this could have any effect on car gas consumption ? currently, in city it is
doing 21 to 23 mpg.
Few months before check engine light started coming on, automatic transmission broke done. Honda did replace it with used transmission, free of charge. Although gear shifting is OKAY, on speed up, it seems that engine is putting more power, I hear more engine noise.
Any comments ?
Please cc me as I am not on the list.

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