The few available range-extended electric car models give drivers added flexibility by using gasoline engines as onboard backup generators.
But using the range extender too often may consume a significant quantity of gasoline, somewhat offsetting the purpose of owning an electric car for some buyers.
Electric-car advocate Tom Moloughney faced that dilemma about two years ago, when he decided to buy one of the first BMW i3 electric cars in the U.S.
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After much debate, he chose to get the range-extended i3 REx, rather than the battery-electric version.
After 27 months and 56,000 miles, he's now confident that he made the right decision.
In that time, the i3 REx has used just over 15,000 kilowatt-hours of electricity, but only 50 gallons of gasoline, Moloughney said in a post on his BMW i3 blog.
2014 BMW i3 REx fast-charging at Chargepoint site, June 2016 [photo: Tom Moloughney]
So far, Moloughney reckons that he's driven on battery power alone 96 percent of the time.
The amount of fuel consumed equates to filling the BMW's 1.9-gallon gas tank about once every month, but in reality fill-ups have been even more infrequent, Moloughney said.
He said he has gone as long as four or five months without buying gas, but has also taken the i3 REx on long road trips that require multiple quick refueling stops.
ALSO SEE: 2014 BMW i3 REx: Range-Extended Electric Car Drive Report, By Very First Owner (May 2014)
Those trips represented the primary use of the car's 0.65-liter 2-cylinder range extender, as it wasn't normally needed in daily use.
The range extender also saw more use during the cold winters in Moloughney's home state of New Jersey, where the drop in temperature had a noticeable impact on battery range.
When it was in use, Moloughney averaged 38 mpg—a fraction below the EPA-rated 39 mpg combined.
2014 BMW i3 REx owned by Tom Moloughney
For the 2017 model year, the i3 gets its first major upgrade since going on sale in the U.S. back in May 2014.
Battery capacity rises 50 percent, from 22 to 33 kilowatt hours, and BMW says the rated range for the all-electric version will be up to 114 miles combined.
The i3 REx, meanwhile, will get an increase in gas-tank capacity.
The tank in every i3 REx has always measured 2.4 gallons, but was electronically limited to 1.9 gallons in U.S.-spec models.
Because that kept the gasoline range roughly equal to or less than the car's electric range, the tiny tank allowed the range-extended model to qualify as a zero-emission vehicle under an obscure California state rule.
While Moloughney has said he can do a refueling stop in less than 2 minutes, many i3 REx owners have "chipped" their cars to open up the extra capacity—a step that won't be needed with the new 2017 version.