A vehicle's design largely determines its fuel efficiency, but how it's driven--and where--can have a significant impact as well.
With their ability to drive solely on electric power at low speeds, hybrids appear better suited to city driving, while diesel drivers have been known to exceed their vehicles' rated fuel economy on the highway.
Note that this study used U.K.-market vehicles, which are rated using the highly-optimistic European testing cycle. The specific models surveyed also aren't available in the U.S.
Volkswagen Golf BlueMotion (2015 VW Golf body style in U.S.)
It was 7 percent for diesels, but only 3 percent for hybrids--which makes sense considering that many hybrids have higher rated city mpg than highway mpg, while it's usually the inverse for diesels and other vehicles that rely solely on internal-combustion power.
Researchers said any advantage hybrids might have evaporated in highway driving, where diesels appeared much better suited to the conditions.
While the U.S. EPA test cycle is tougher than the European standard, those results likely won't surprise U.S. drivers.
Citroen C4 Cactus
Hybrids did have an advantage in another area, though.
Researchers noted that hybrids produce consistently lower levels of nitrogen-oxide emissions than diesels, especially in urban driving, when some zero-emission driving on electric power is factored in.
So while diesels may return more consistent fuel-economy results, hybrids remain the cleaner choice.