Should I fill up my gas tank when the tank is half-empty?

Serious question: I'm trying to save the max amount I can on gas (aren't we all). I typically fill up once a week, regardless of how full the tank is. It's usually between a quarter and a half a tank when I fill up. Is this a mistake? Because I know a full tank adds weight to the car, and extra weight... show more Serious question: I'm trying to save the max amount I can on gas (aren't we all). I typically fill up once a week, regardless of how full the tank is. It's usually between a quarter and a half a tank when I fill up. Is this a mistake? Because I know a full tank adds weight to the car, and extra weight brings down gas mileage. But the other side of the coin is that if I wait a few more days to fill up, gas prices will likely be about 5 cents higher, anyway. I'm looking for serious answer: If I'm trying to save money, should I still fill up when the tank is half-full, or should I wait til it's closer to empty?
Update: Good advice Mrs. Tech. I'm not a car expert by any means, but what you're saying makes sense. I'll keep doing what I'm doing.
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