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I have 13.4 gallon tank, and I always fill all the way up and wait until I get the empty light before refilling.
Ideally though, the less weight you drive with the better. Some people even mod their cars to have a smaller tank, so they forcibly run with less weight on a full tank.
I say, just fill 'er up, and unless you're willing to go to extremes, don't worry about it beyond that. You need to use a full tank every time you fill up to accurately and easily calibrate your Scangauge, anyway.
Once you get to a point where you're getting twice as much MPG as stock, then you can start half-filling and maybe thinking about getting a half tank or something.
If I had a garage and was getting really bored, I might consider syphoning my excess gas into a 50-gal drum but until then...
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