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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:

  1. Low gear - slows down car, onboard computer reports fuel consumption = 0.
  2. High gear - speeds up the car (faster than neutral), onboard computer still reports fuel consumption = 0.
  3. 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?

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When you use engine braking to go downhill, the car ecu can reduce the amount of fuel needed to keep the engine running - even no fuel in some conditions. So the onboard computer can report 0 fuel consumption.

Also the computers are not perfectly accurate, see my answer here: https://mechanics.stackexchange.com/a/76400/10976

Once you let the engine idle, the fuel used is that amount required to keep the engine running, but if you are doing 70mph downhill then the mpg will be reported as per the calculation of distance and fuel used, which again is not representative compared to driving on the flat or driving up hill.

The "best" way to drive is to have full control of the vehicle. So, coasting out of gear is not advised. I Had a customer that did that - against the advice of several people. One day we had to go and get his vehicle as he had been coasting, needed a gear and at 60mph put it in second (instead of 4th) and he broke the crankshaft... He never coasted after that - expensive lesson though.

As for good fuel economy, excluding the vehicle, start with properly serviced, tires and pressures set and drive gently avoiding harsh acceleration or braking - anticipation of changing road conditions is key.

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Maximize economy by not using the wide pedal in the middle ( aka brake). It converts kinetic energy to heat which is all wasted. Such as ,take your foot off the gas when you see a stop sign, don't drive up to it and slam on the brake. It also saves wear on the brakes; my car has over 80,000 miles on the original pads and they will easily go past 100,000 miles.

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If the car computer is reporting zero miles per gallon, it is due to the math involved. Most cars will stop injecting fuel above a certain rpm when the "throttle" is closed, as part of economy design, but x number of miles per zero gallons is dividing by zero. Instead of displaying an error, it shows 0 mpg.

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