The impact that they have on fuel economy, (If any), might be based on just how you have them set...
You can adjust them so that most of the airflow passes through them with minimal re-direction...
Or you can get those barn doors swiinging wide...
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2010 Blue RT Audio & Convenience model Se-5 transmision. Dealer-added foglights, accent LEDs and extra speakers. Some RTS chrome windshield, front spoiler and top trunk trim, RTS frunklight and frunk liner, Top Trunk liner, Chrome lower air deflector, fendertip LED bulbs and ISCI floorboards added by yours truly...