But from a purely fuel economy perspective, you can see that hybrids and “extended range” vehicles like the Volt are not delivering really low fuel consumption numbers (actually, I’ve seen 20 percent better than the Volt from a Volkswagen Passat Diesel, and even Chevy’s Cruze Diesel can do better). So if you want “killer” fuel economy, you’ve got to go electric.
I'm not sure if you're going for a different point, but I think you're missing the purpose of the Volt (or at least your article doesn't suggest it).
TYPICALLY (not absolute), Hybrids will work better in city driving, Diesels on highway jaunts, and range-extended hybrids will be the best available solution, ASSUMING YOU HAVE ACCESS TO CHEAP/FREE ELECTRICITY AT YOUR DISPOSAL.
5.7L/100km is the consumption IF the battery in the Volt is depleted (well, with its reserve always there...). So, while I'm sure you saw better in diesels on the highways in varying conditions, etc., the Voltec powertrain is pure genius and should be the 'way of the future'
for certain applications.
If there was one and only one "true" best way forward, there would not be such variety.