On passenger the road #2 diesel vehicles usually saw high 30s to low 50s in MPG through the early 2000s. They did great on fuel economy. Even light-mid duty diesel engines are "good" on fuel economy. We can currently store more energy in a 40 gallons tank of diesel fuel than any battery technology for the same volume. You'd have to include the volume used by the rest of the powertrain, which is fair, to be equivalent.