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My exhaust manifold had a small crack in it where it connected to the tailpipe. Not really loud, but exhaust was making into the cabin and giving me a headache on long trips. So could be the manifold itself - about a $500 part or its caskets.
That said, unless the leak is large, I wouldn't think it would effect the o2 sensor much. In my case the only difference was cabin air quality, not MPG. What was your MPG prior to this? Bad MPG can be caused by bad O2 sensor you need to test it to be sure. On my 240 testing the O2 sensor is an easy test with a voltmeter for you or your mechanic. I think is is also on the 850.
Bad MPG also has lots of other possible causes. If you are hearing more of a hiss then a throaty blow you might have a vacuum leak. A dirty throttle body or a crack in one of the intake air hoses would be a bigger effect on MPG then an exhaust leak. Other fun things to look at - timing, filters (fuel, air, oil), plugs, dist cap rotor... basic tune up things.
Good Luck!
My vehicles -
'88 245 auto 147,000 miles & about 24 MPG
'93 850 auto 282,000 miles & about 27 MPG
'94 855 manual 181,000 miles & about 30 MPG
'97 2500 Dodge Diesel ex-cab truck 128,000 miles & about 21 MPG
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