I had the cat converter changed last week; since then, the best city milage I've gotten is ~14-15mpg. Before the converter was changed, I was getting between 17-18 city and 21 on the highway. I haven't had it on the highway since the change. The car is a MT with ~3000 miles on the odometer. Should a new converter change my mpg that much

 

What was the reason for the change? I'd give it some time and see if it improves.

 

CEL came on and dealership said the cat "wasn't working correctly" so they replaced it....I hope you're right about giving it some time.....

 

give it some time, it should improve. what work have you done to your car?

 

If you mean upgrades, the M flash was done right after I got it; other than that, and an oil change, nothing has been done to it.....