I'm estimating 81 mpg at best, and 47 mpg at worst.
A friend is getting well over 100MPG. But his journey is just about optimum for the car. Plugs in at both ends.
How big a car, carrying what, on what roads, in what traffic? Far too many variables to compute.
Probably - with serious optimization - about the same as from gasoline/electric hybrids, maybe a little worse. Most people are as surprised as I was to find out the maximum theoretical efficiency of a diesel engine is about 50% while the maximum theoretical efficiency of a gasoline (otto cycle) engine is about 65%. It has to do with the diesel adding heat at constant pressure while the otto cycle engine adds heat at constant volume. Anyway, gasoline engines typically fall behind because the theoretical maxima are at infinite compression ratio with perfectly insulated cylinder heads, cylinders, and pistons. Diesel engines are capable of much higher compression ratios than gasoline engines, and the method of throttling gasoline engines has the effect of reducing the compression ratio even more. That has given diesel engines a reputation for efficiency and they are more efficient than turbine engines (about 40% max) and rockets (about 12% max). However, hybridization has allowed gasoline engines to reach efficiencies that are not very practical for diesels. The current generation Prius spends a lot of engine operation at 38% efficiency - the 2016+ is supposed to reach 40%. Diesels can't compete; although that represents only 61% of the theoretical max for gas engines it is 80% of the theoretical max for diesels, and that last bit of efficiency is the very devil to achieve. It would require entirely new materials and other major improvements. As the source says, Conventional gasoline engines have long run about 25 percent, diesels at 30 to 35 percent.
