Electric cars still cost significantly more to buy than gasoline-powered vehicles of the same size and capabilities.
But cars that plug in are almost always cheaper to run per mile, often by a significant multiple.
A study released Thursday has quantified those differences, comparing the cost-per-mile of gasoline versus electricity in different states.
Using state-by-state electricity-cost data from October and gas prices from late December, Michael Sivak and Brandon Schoettle at the University of Michigan Transportation Research Institute calculated the breakeven per-mile cost between the two vehicle types.
On average across the U.S., the study concluded, a gasoline vehicle would have to return 57.6 mpg to cost as little per mile to run as a plug-in electric car.
Only one vehicle without a plug this year has an EPA fuel-efficiency rating that high: the 2018 Hyundai Ioniq Blue Hybrid, at 58 mpg combined.
The numbers varied hugely by state, though. In Hawaii, where both electricity and gasoline are more expensive than the national averages, the breakeven was a gasoline car at only 34.1 mpg.
That’s still far above the average of about 25 mpg that new cars have maintained from 2014 through 2017, as calculated in a different report by the same institution.
In Washington state, on the other hand, the gasoline vehicle would have to return 90.0 mpg to be as cheap as an electric car—and no vehicle on sale today reaches anywhere near that. Without a plug, anyhow.
Overall, a gasoline car costs 2.3 times as much to run for the same number of miles as an electric car for the U.S. as a whole.
The number is lowest in Hawaii, at just 1.4, and highest in Washington, at 3.6. Following Hawaii on the most-expensive list were, in order, New Hampshire, Connecticut, Rhode Island, and Massachusetts.
The highest ratio was in Washington state, at 3.6, followed by Oregon, Idaho, Louisiana, and Utah.
Various state-by-state conditions explain many of these disparities.
California, where roughly half the nation’s plug-in cars are sold today, sits in the middle of the various rankings, because its gasoline is highly taxed but it also has relatively expensive electric rates.
Washington state has exceptionally cheap electricity; some residential customers in certain areas pay as little as 3 or 4 cents per kilowatt-hour, against a national average of about 12 cents.
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While the ratio between the most and least expensive gasoline in the country was roughly 1.5, the priciest electricity rate was 3.0 times that of the least expensive.
The study used NHTSA data on the distance driven by light-duty vehicles, reported last year at a national average of 11,443 miles.
It assumed that average light-duty vehicle also returned 25.0 mpg, per the earlier study, and an EPA average energy use for all plug-in electric vehicles of 33.0 kilowatt-hours per 100 miles driven.
An abstract of the UM-TRI study is available online, though the full study is not presently available to the public.