i'm planning on getting a muffler soon for my civic ex.

the stock inlet is 1.75". the new muffler will be 2.5" so my question is how much more more gas consumption should i expect to waste if i expand my piping to 2.5". i'm currently getting about 450km/tank(33mpg).

 

better flowing exhaust will actually decrease fuel consumption.
The problem is, just like an intake, everyone likes the sound of it and pushes harder, which consumes more fuel.

BTW, if you're only increasing the size of the muffler inlet and not the entire system than it's pointless.

 

look get the whole exhuast done and you'll improve fuel consumption. the engine will breath easier with bigger pipe.

 

isnt it a must to expand the piping if your inlet is 2.5"?
i'm not really trying to get it to gain any power because my civic is already slow enough. I just want the nice low tone muffler and looks.

 

you don’t have to.... you can expand the pipe to fit right at the muffler, or even slip the smaller piping into the larger and weld it in place. Crude but you'd be surprised how often it's done.

 

just drive at a steady speed n your mpg shouldnt change