I drive a 4,0-litre Jeep Wrangler with the six-speed manual gearbox. When I drive at 120 km/h, I can either select sixth gear at 2 000 r/min using 75 per cent of the throttle travel or fifth gear at 2 500 r/min using only 50 per cent of the throttle travel. Which gear will give me the best fuel consumption (my vehicle has no fuel-consumption readout)? My brain tells me the more I depress the throttle, the more fuel the vehicle uses.
HERMANN BEKKER, Douglas
Answer: Your assumption that your vehicle uses more fuel when you depress the accelerator pedal further does not hold true when comparing the fuel consumption using different gear ratios at a set speed. Yes, the throttle restricts the airflow to a petrol engine and therefore limits the amount of fuel that can be added at the ideal (stoichiometric) air-fuel ratio, but there are many other factors in play, including engine speed. Brake-specific fuel consumption (BSFC) maps show the specific fuel-consumption regions of an engine at all speed and load points (see picture). The spots on the map at which the engine-speed load points fall in fifth and sixth gear will indicate which is more fuel efficient. At 120 km/h, your vehicle needs a fixed amount of energy to maintain a constant speed. The engine delivers the same work to maintain 120 km/h in fifth and sixth gears. The question is which method is more efficient in converting the chemical fuel energy to motive force. Our gut feel is sixth gear because the lower engine speed results in less friction and the higher throttle opening will lower pumping losses.