[anamexis]

That may well be, but it seems like the 15% is a high-level number that takes a lot of factors into account, such as power used by heating accessories, and the fact that cold air is denser (more air resistance).

https://www.fueleconomy.gov/feg/coldweather.shtml

My original wording was inaccurate, as it's more about ICE-powered vehicles than the engines themselves.

[xyzzyz]

That sounds reasonable: note that I said that on short routes the engine might not have a chance to warm up.

Your source, however, quoted this 15% figure for city driving, I think that’s worth stressing as well. I wonder if they have any data for highway driving. I’m too lazy to Google, though.