It's one of the big unknowns about plug-in hybrids: How much do they actually get plugged in?
Battery-electric cars have to plug in to remain usable, but plug-in hybrids and range-extended electric vehicles don't.
And while some makers--most notably Chevrolet and Ford--happily release data showing that their drivers plug in more than expected, other makers are silent.
DON'T MISS: Chevy Volt Owners Drive More Electric Miles Than Nissan Leaf Drivers: Why? (Nov 2013)
The labs lead Advanced Vehicle Testing Activity for light duty vehicles on behalf of the U.S. Department of Energy.
2013 Ford C-Max Energi plug-in hybrid, Marin County, CA, Nov 2012
The data was collected and analyzed from on-road usage encompassing 158,468,000 total vehicle miles by 21,600 vehicles.
The cars included three battery-electric vehicles as well: the Ford Focus Electric, Honda Fit EV, and Nissan Leaf.
The five other cars were the Chevrolet Volt, the Energi plug-in hybrid versions of the Ford C-Max and Fusion, the Honda Accord Plug-In Hybrid, and the Toyota Prius Plug-In Hybrid.
Average monthly Vehicle Miles Traveled on Electricity [Idaho National Labs, SAE 2015 World Congress]
And as the data presented by the labs at the recent Society of Automotive Engineers World Congress shows, the electric miles traveled by those five were roughly proportional to their electric range.
The plug-in Prius, with only 11 miles of electric range, averaged 207 electric miles out of 1,261 total monthly miles.
ALSO SEE: Ford Plug-In Drivers Do 60% Of Miles On Electricity, Just Like Volt (Aug 2013)
The Volt, on the other hand, averaged 759 electric miles out of 1,020 each month--roughly in line with Chevrolet's own data from the tracking of Volt travel it does via the Onstar system.
Volts for the 2011 and 2012 model years were rated at 35 miles of range, rising to 38 miles for the 2013 and 2014 model years.
Overall Vehicle Miles Traveled on Electricity [Idaho National Labs, SAE 2015 World Congress]
Interestingly, the Volt's average of 759 monthly electric miles was close to the totals of the three all-electric models: 808 for the Leaf, 807 for the Fit EV, and 796 for the Focus Electric.
The pair of Fords and the plug-in hybrid Honda came in the middle: 339 electric miles of 1,033 for the C-Max Energi (now rated at 20 miles of electric range), 361 of 1,033 for the Fusion Energi (also 20 miles), and 278 of 1,249 for the Accord Plug-In Hybrid (13 miles).
These results match what might be expected: The more electric range you have, the more electric miles you can drive.
2011 Chevrolet Volt plugged into Coulomb Technologies 240V wall charging unit
But they do point out that the 11-mile plug-in version of the Prius hybrid is by far the least electric of all the cars with both battery power and engines, closely followed by the 13-mile Honda Accord Plug-In Hybrid--while the Volt is by far the most electric.
Ford and General Motors have provided data on the electric miles driven by their cars, as noted in the articles linked above.
Toyota, however, refused to provide that data when Green Car Reports requested it some months ago.
[hat tip: Max Looker]