gas mileage ?

Can an exhaust leak cause your MPG to go down. If so just how far can it bring it down.

Yea it can. It can make the O2 sensor think the engine is running too lean and the pcm will dump more fuel in to compensate for what it thinks is a lean condition. It will MURDER your gas mileage if its a good leak.

it will also lack power, and cylenders close to the leak will run hot enough that you can accually blow a hole in the piston itselfe, exhause leaks by or at the manifold should be fixed asap