I installed a magnaflow xpipe with resonators to my already installed solo axlebacks and now I'm getting wierd mileage. Before the install the car was averaging about 21 mpg all the time. I checked the mileage the last two days on the way home from work on a trip of about 30 miles. In normal mode with dod kicking in I averaged 18.8 mpg. I tried it today in manual mode with same driving route and got 20.8. Any ideas what could cause the mileage to actually go up without dod?