I recently bought my 99 Civic Si about 3 months ago. I was getting about 260-280 miles to the tank. So about 2 weeks ago I bought a Greddy TIC exhaust to swap out the Greddy Evo2 it had on it. Anyways so while installing it I noticed it had an after market high flow catalytic converter. The Evo2 resonator pipe was chopped in half and a piece of piping was welded on to it that was welded to the cat as well. To make a long story short I tracked down an oem cat to replace it. So with that said, now im barely clearing 200 miles to the tank. What gives?? Was the high flow cat giving me the extra MPG? Any help would be appreciated.

 

Did you change the Inside Diamater of the ehxaust? Reduced flow on the exhaust end could hurt your gas mileage, If you have good intake flow and not enough exhaust flow your gas mileage will go down, I'd replace the stock cat with a high flow cat and see if that helps.

 

I think the TIC diameter is the same if not a little bit bigger. I will check that out tomorrow, I still have the old EVO exhaust in my garage.

 

Any other ideas?

 

You should be getting even more than 260-280 unless you drive hard all the time, so 200 is really low. Don't know the solution though.

 

I really don't push my car hard unless I have to. I'm gonna try changing a few things next week.