Will leaving it broken kill my MPG or can I leave it with the 1/2" gap between the exhaust manifold and the pipe?

 

Just drive your car to find out. It really depends on the exact location of the gap, if it is by the o2 sensor then your ecm will probably get inaccurate a/r ratio readings and thats when problems occur.

I personally had a inch long gap in my y-pipe just 2 inchs after the y connection and my gas mileage didn't suffer much. However my performance faced a decrease after 3500 rpms at WOT.

Pat

 

Yea, mine leak is right where the pipe connects to the manifold. It sounds pretty cool though, when I'm driving between buildings in ally ways... I think I'll leave it..