The fact that diesel has more BTU's per gallon vs. gas is irrelevant. It takes more crude to produce one gallon of diesel than it does one gallon of gas. What is being refered to here is energy density of a fuel and not energy efficiency of a motor - the amount of work you can get out of a unit of energy.

If all are right the most "efficient" would be a hybrid diesel car. I am not aware of any of these units being produced - I am sure there is a good reason althought it may not be engineering / mechanically related.

In countries where gas and diesel are more expensive (as a proportion of disposable income - not the US / Canada) the country's fleet of vehicles tends to have smaller engines (few have more than 4 cylinders), they tend to have more turbo diesel passenger cars and many very small gas engines < 1.0 liter - 850cc passenger car engines are common. This says to me that when it comes to economising smaller = less expensive.

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