Although it is well known that diesel engines are comparatively powerful and fuel-efficient, it is that latter characteristic that really come to the fore in a recent undertaking by uber-hyper milers Helen and John Taylor.
Consider: they climbed in a Volkswagen Passat TDI SE (manufactured in Chattanooga, TN). The car--a regular consumer model, not something specifically set up for what the Taylor's undertook--features an in-line four cylinder diesel engine that produces 140 hp and 236 lb-ft of torque. There is a Selective Catalytic Reduction (SCR) system as part of the exhaust system that assures that what comes out of the tailpipe is clean enough to meet the regulations in California (and the other 49 states, as well). (You may have heard of some vehicles being referred to as "Clean Diesels"; this is one of them.). The Passat is equipped with a six-speed manual.
While regular drivers can achieve 43 mpg highway with the car, for the Taylors that would be profligate.
On May 3 they departed Houston with 120 lb. of luggage in the trunk and a full tank of ultra-low-sulfur fuel. They drove during daylight hours and for no longer than 14 hours per day.
On May 5 they arrived in Sterling, VA.
That's 1,626.1 miles later.
That's a new record of 84.1 mpg.
(The previous record was 1,526.6 miles, set in Europe in a Passat 1.6 BlueMotion diesel.)
So even though you may see the price of a gallon of diesel being hirer than regular, there is nothing regular about the fuel efficiency that the compression-combustion diesel can provide--even if you don't drive like the Taylors.