would i be able to get regular gas mileage and waht not w/ one on?

my cat is gone and i'll be using a test pipe till i can source out a new cat, or maybe use it everytime before and after passing a smog?

also would putting a test pipe throw a cel? how would you come about using a test pipe normally

thanks

 

i didn't notice any difference with gas mileage. it all depends on driving style. it depends on the year whether it will throw a light. anything with obd2 will throw one cause it will be missing the down stream o2. i run mine all year untli i need to pass emissions then i thorw it back on