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I've done a spreadsheet analysis using the gas prices from last year, during which there were several wild swings in the prices. The analysis shows that following a strategy of buying gas immediately prior to prices rising, and delaying purchases while prices are falling or stable does result in savings. There is no "averaging out"! The analogy to the stock market, where dollar-cost-averaging is valid, is flawed.
The problem of course is: while there are savings, the savings do not amount to much, especially if you have a small gas tank or high usage or low mpg that do not allow you to benefit from the downward price trends due to needing to fill up too soon to take advantage of them.
Anyways, with my Insight (10.5 gal tank) and 240 mi weekly commute, I can wait two weeks before filling if need be, so one would think my annual savings would be substantial, but they're not of course. Assuming that I had been able to perfectly predict peaks, both high and low, last year, my savings would have been no greater than $3.82 (vs. filling once per week regardless of price).
With a larger tank, or applying the strategy with multiple cars, the savings can be greater, of course, but nothing earth-shattering. For example, the ability to wait two weeks before filling a 40 gal tank would result in savings of about $15.00
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