Better Gas Mileage??
 

I get avg 31 city/hwy with the a/c on. 98 Civic coupe, auto. I have heard that a K&N air filter, platinum plugs, and 2" exhaust all the way out will help mileage. My car has a 2" through the cat and a round muffler, back to about the Right Rear wheel - where it has a bolted flange down to a 1 1/2" pipe and a second muffler. Why is this? Would I be better off to get rid of the 1 1/2" and the second muffler and go with 2" and a resonator tip? I don't want the thing loud at all but a low mellow sound would be ok if I got 4 or 5 more mpg... And if so - do they sell the pipe and resonator tip that would just bolt on to that flange?

Gas mileage - yours is not too bad for an auto. If you are concerned, just do a tune up (basic NGK plugs, wires, cap, rotor, ignition timing). Any CEL codes?

No codes. New cap and rotor and plugs (5k miles). I have just the regular NGK plugs in it. Heard that platinums are better? And I can't figure out why the exhaust goes DOWN from 2" to 1 1/2"? That seems like a big restriction to me...

Regular NGK plugs are best and more cost effective for your car.

The stock exhaust is 1.75", right? So it's really no restriction.

I didn't know what the stock exhaust size was? This is the first Honda and the first small car I have ever owned. Been used to "American Iron" trucks for many years, and I have a Chevy Big Block Nova as well. These little cars are new to me. I love the little thing and the idea of getting triple the mileage of my former daily driver truck is intriguing and kind of a challenge. I got flamed on the other Civic forum just for asking this question - so thanks for the honest replies...

Which forum?

I edited information in post #4. The stock pipe diameter is 1.75". If yours is close to this, then you are fine.

Honda Civic Forum I had a guy call me a "troll" since my screen name was "racinfan" and I was asking the question I asked. Seems wrong to me - but I am used to a Nova and a Dodge forum where everyone is respectful for the most part. Had the screen name for 10 years now.. Just kind of rubbed me the wrong way...??
I might have measured wrong but I am pretty sure that I have 2" back to the 1 1/2" step down at the flange. I think I will take it to my custom exhaust guy that has always done my truck and Nova and have him see what he thinks. My philosophy has always been "bigger and less restrictive is better" when it comes to exhaust on my other stuff. Just kinda new to 4 cyl and foreign cars - and wanted to see what you guys thought....

Flaming is ridiculous. You'll find this smaller forum to be more kind. Click the report button if you ever feel that you are being harassed.

Your Civic engine is designed to run optimally on a 1.75" exhaust pipe. There's no benefit whatsoever in increasing the diameter of the stock pipe.