Toyota Repair: Replaced catalytic converter gives worse gas milage, correct diagnosis, speed ranges

Question
Sir:  I have a 1997 Toyoto Tacoma.  My check engine light came on and the local Toyota computer check said I need the
catalytic converter replaced.  I had it replaced for a cost of $860.  Before this I got around 20 miles to the gallon.  I have only driven around 100 miles since replace ment but am getting around 11 miles to the gallon now. What has happened?   Thanks for any advice!  Roy

Answer
Without knowing the original diagnostic trouble codes I have no way of knowing if that was the correct diagnosis or the reason why the converter was replaced, does the truck seem to have normal power through all speed ranges? Try to let me know what the trouble codes were before the converter was replaced.