Wyoming (39.7 percent), Washington, D.C. (35.3 percent) and Maryland (34.8 percent) were next in line with the largest variance in rates. Conversely, South Dakota had the smallest difference at 1.9 percent, followed by Arkansas, Utah, Iowa and Kansas, which were all below 6 percent.
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Complicating the issue is that there is no rhyme or reason to why—or even when—the rates are so disparate. The national median cost of a new full-coverage auto policy for a 35-year-old single male driver with a $500 deductible and a clean driving record was 7.5 percent cheaper in December than in March, according to the website.
"What's most interesting is that there's no discernible pattern to detect here," said Mike Barry, spokesman for the nonprofit Insurance Information Institute, in a released statement.
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In the aforementioned case of Hawaii, it was nearly 48 percent cheaper in December than it was in March. December is often the least expensive month for insurance because new regulations or laws that could raise prices often go into effect at the beginning of the year.
That said, December was the most expensive month in 11 states, the study revealed.
"It appears as though the trend lines are all over the place, which doesn't really lend itself to a clear-cut reason or hypothesis for why that's the case," Barry said.