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Brake specific fuel consumption (BSFC) is a measure of an engine's efficiency. It is the rate of fuel consumption divided by the rate of power production. BSFC is specific for the piston engine known as the reciprocating engine. The general term is specific fuel consumption (SFC). There is also thrust specific fuel consumption (TSFC) for turbine and rocket engines.

To calculate BSFC, use the formula BSFC = Fuel_rate / Power
Where:

Fuel_rate is the fuel consumption in grams per hour (g/hr)

Power is the power produced in Kilowatts where kW = w * Tq / 9549.27

w is the engine speed in rpm

Tq is the engine torque in newton meters (N·m)

Note: The Power in the BSFC calculation is not weather corrected.

The resulting units of BSFC are g/(kW·h)
The conversion between metric and U.S. units is:

BSFC_US(Lbs/(HP*Hr)) * 608.277 = BSFC_METRIC(g/(kW·h))

BSFC_METRIC(g/(kW·h)) * .001644 = BSFC_US(Lbs/(HP*Hr))

To calculate the actual efficiency of an engine requires the energy density of the fuel being used.
Different fuels have different energy densities defined by the fuels lower heating value.

Some examples of lower heating values for vehicle fuels are:

Certification gasoline = 18640 BTU/lb = .01204 kW·h/g

Regular gasoline = 18917 BTU/lb = .0122225 kW·h/g

Diesel fuel = 18500 BTU/lb = .0119531 kW·h/g

Thus a diesel engine's efficiency = 1/(BSFC*.0119531)
and a gasoline engine's efficiency = 1/(BSFC*.0122225)

A typical cycle average value of BSFC for a gasoline engine is 322 g/(kW·h). This means the average efficiency of a gasoline engine is only 25%. A reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine speed is at around 2000 rpm. For a gasoline engine, the most efficient BSFC is around 256 g/(kW·h) or an efficiency of 32%. Efficiency is worse at other operating conditions. As you can see above, lower values of BSFC mean higher engine efficiency. Diesel engines are more efficient than gasoline engines. A diesel engine can have a BSFC as low as 199 g/(kW·h) and around 42% efficiency.
See also Fuel economy in automobiles.