horrible gas mileage after longtubes

The car is not dyno tuned yet so this may be the cause. Have you guys seen a decrease in gas mileage after installing longtubes? The car has a Modular Powerhouse mail order tune in it, but that's the tune i had in the car before the longtubes. It will be dyno tuned on Friday by Bob Kurgan. So far on my first 1/4 tank I got 35 mi in the city. Before the longtubes I used to get 45 mi.
 
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