Well, I don't know how big a hole you had but it certainly is possible to see some increase in power and fuel economy if you can reduce or eliminate the backpressure caused by the exhaust system. Having said that if you regularly get 35-37 MPG you are doing quite well and the 45 was probably a fluke, but I could see an extra 10% not being out of the question...so maybe 40? Also, your lower mileage now might be cause not by the new exhaust system but more likely from the effects of winter blended fuel.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz