I have a 2004 Chevy Tahoe, 5.3 with 190,000+ miles on it that me and my local mechanic are going to install and do a controlled test on the exhaust and the gas mileage. It always averages 15.4 MPG and we will see if it goes up from there over time. If I do not get any results, I will ask for my money back, but I expect that won't be necessary. We will see.
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