So lean times are upon us, and I started paying more attention to my car's onboard computer saying what the MPG is. I have a couple of questions...
Does engine braking really save fuel? The car states that the MPG while going downhill, foot off the gas and using a low gear is 0. If I put the clutch in (i.e. neutral), the MPG rises to a small but finite value (engine idling value).
In an ideal world the car should detect that work to compress the air in the cylinders is being supplied by the car going downhill, therefore it should shut off the fuel injectors. However, in my experience, going downhill in 6th gear actually speeds up the car compared to going downhill in neutral. Which means that fuel is being injected and the engine is generating power.
To summarise the above observations, when the car is going downhill:
- Low gear - slows down car, onboard computer reports fuel consumption = 0.
- High gear - speeds up the car (faster than neutral), onboard computer still reports fuel consumption = 0.
- Neutral - the basis of comparison for both above, onboard computer reports idle fuel consumption.
As a rule of thumb, is driving in as high a gear as possible the most fuel economical way to drive? If I wanted to accelerate from a low speed, say 30 km/h, but don't mind the acceleration time (say I am in a clear road and in no rush), from a fuel economy perspective, is it better to floor the accelerator in 5th gear, or to apply moderate gas in 2nd gear?