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I'm curious how various performance mods have affected mileage on people's cars. My last vehicle was a Nissan Maxima. With a cone intake and Magnaflow catback I was running 190 hp (a gain of about 10-12 hp) but average mileage dropped from 24 to 21 mpg. I'm getting 30-31 average in my 2007 EX 5-speed now. I'd hate to do similar mods on my new car and end up getting mileage not much better than the Maxima, but from a much slower car (the Maxima turned 15 flat at the track even with really bad launches).