Diesel contains more energy per gallon than gasoline. A typical comparison lists diesel at about 128,700 BTU per U.S. gallon, versus roughly 115,400 BTU for gasoline.
The main reason is density. Diesel fuel is heavier and packs more mass into the same volume, even though gasoline and diesel have similar energy values by mass. Diesel’s net heating value is about 43.1 MJ/kg, compared with approximately 43.2 MJ/kg for petrol; its volumetric advantage comes largely from its greater density.
This does not mean every diesel vehicle automatically travels farther on a gallon. Engine efficiency, vehicle weight, gearing, driving conditions, and fuel formulation matter too. Diesel engines also use compression ignition rather than spark ignition, and their higher thermal efficiency helps explain their strong fuel economy.
A common mix-up is confusing energy per gallon with octane rating. Gasoline is designed around octane and spark ignition, while diesel fuel is evaluated chiefly by cetane number and compression-ignition behavior.