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You aren't going to get more light with less wattage, no way no how. A normal halogen is 60W high beam, 55W low beam. You can buy bulbs with higher wattage, but only if your wiring can handle it.
Either way, it won't have a noticeable effect on gas mileage. It takes 745watts to make 1hp and a normal car is probably using 20-40hp at cruising speed.
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