My daily driver is a bone stock 2000 Civic coupe 1.6 auto.I have been getting about 300 miles a tank using 10 gallons with the gas light on once in a while.I replaced my cat with a test pipe i got thru a trade years ago and got the following results:

330 miles to the tank and no gas light and when refilled took the same amount as usually(9-10 gallons) 87 octane.
Random check engine light(maybe once every two days and i simply reset it with my Obd2 scantool)The code is catalyst related due to the obvious.

My car is full power everything and even extra weight in the trunk(music and tools).Maybe monthly i check my tire pressure and my driving habits havent changed a bit and i always have my A/C on.

So in conclusion i thought cats were supposed to help the environment?There goes that theory out the window with my car.

DISCLAIMER:These results happened with my car it may not apply to yours.