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You will not lose much gas mileage at all if you stay off the pedal. Maybe 1-2 mpg, but then again the cai is supposed to help with gas mileage so they could equal out.
I think it depends at what speed you cruise at. If you drive alot on the highway doing 90+mph with the stock 355 gears and now go with 410 gears and still cruise at 90 mph you will see a big differance.