It works out at €1.40 a litre, or £1.20 a litre.
Diesel at the local garage I passed at lunchtime is £1.89 (57% more). The web [0] suggests that is more expensive than anywhere in europe other than France and Denmark for some reason.
So nowhere near "half as cheap". Throw in poor American average mileage and I suspect the price-per-mile is quite similar.
Diesel in Norway was about €2.5 a litre today.
Diesel engines in the US are used for things where they do count fuel costs. (there are a few diesel cars/SUVs, but they are a tiny minority). In short for most diesel users the efficiency is very similar between US and Europe.