Muffler and MPGs/Air intake

I have a 1991 honda civic si 1.6.

My OEM muffler rusted out and i replaced it with a cheap $29.99 muffler from auto zone. My OEM muffler had 2 outlets and my new one only has one.Before the OEM muffler went i was getting around 37-42 mpg now 30-35.So is my new muffler causing this or should i be looking somewhere else (spark plugs etc)?

Also will a cold air intake inprove mpg?

Do you drive aggressively and at maybe over the speed limit? If so then a restricted exhaust or intake can reduce mileage, at least some, because of breathing problems. Most modern cars, unlike the pre-80 models, have very good breathing, but econo models that are using close to full throttle for normal driving are different.

Someone who knows your car better will have a more authoritative answer than mine, but I would guess you may recover that mileage with a freer flowing muffler and you might (less likely) gain a little with the air intake mod.

One symptom of a choked exhaust is an engine that is totally gutless in the upper rpm range but seems kind of normal at low rpms.

The biggest restriction by far that airflow experiences in an engine that is powering a car that is obeying speed limits is the almost closed throttle.

Your new muffler could definitely reduce mileage if it is restricting the exhaust compared to stock. On the other hand, a cold air intake will do next to nothing for mileage, but might slightly improve power at wide open throttle.