Ive got an 02 SD v10. Running without a catalitic converter SEVERELY decreased my MPG. I was getting about 150 miles to a TANK of gas without a catalitic converter. The exhaust reaked like the engine was flooding all the time also. Didnt gain any extra "performance" at all. Just horrible fuel economy and poor emissions. I also forgot to mention the truck wouldnt hold a steady idle minus the catalitic converter. Ive got an aftermarket cat in it now, but as soon as I can afford it, a new Ford factory one is going back in it.

 

wow good information here.. thanks for all the post's, im definitely going to leave the cat on it on and start looking at different mufflers. Looking for something that sounds good but not too loud in the cab. Good performance is always important as well.