A BSFC (brake specific fuel consumption) map of a SI (spark ignited) ICE (internal combustion engine) describes the engine's fuel efficiency for every combination of engine torque (usually in Newton meters) and engine RPM. For every driving condition (engine horsepower requirement) there is an optimum combination of engine torque and RPM for best fuel economy. Usually the SI engine is most efficient at about 35% of redline RPM and 75% of maximum torque. For the 2.5L engine in my 2003 Subaru Forester with a 5-speed manual, the maximum efficiency is at about 120 Nm and 2200 RPM (27.6 kW or 37 HP). City driving usually requires less than about 10 HP (easily calculated for steady speeds). For power requirements below about 30 HP, the BSFC map shows that the optimum engine speed for best fuel economy is below about 2000 RPM. I usually get into 5th gear between 25 mph (1100 RPM) and 35 mph (1500 RPM), depending on driving conditions. I consistently get 38 to 39 mpg in the summer (34 to 35 mpg in winter) and drive over 500 miles between fuel stops. Do not be afraid of using torque up to 80% or 90% of WOT (wide open throttle). The engine computer will optimize the spark timing to prevent pinging and engine damage.