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Some modern cars, including models by BMW, Ford and Audi, have a system on the panel that tells the driver the next gear to use while driving, for optimal fuel economy.

How does the vehicle decide which gear provides the best fuel economy? What data does it require to make that indication in real time?

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  • $\begingroup$ I don't know but modern cars measure all kinds of things: Wheel speed, engine speed, mass airflow sensor data, air temperature, maybe torque (not sure about that one). The mathematical model probably uses all of those, and it probably was designed to fit some huge pile of empirical data. $\endgroup$
    – james large
    Apr 29 '15 at 15:53
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    $\begingroup$ Extra credit: "Optimal" for what purpose? Best fuel economy? least wear and tear on the engine? shortest 0-60 acceleration? ... (probably not acceleration, 'cause who's going to have time to watch the shift light?) $\endgroup$
    – james large
    Apr 29 '15 at 15:56
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    $\begingroup$ It depends if you consider them to "work" or not - I don't. $\endgroup$
    – jhabbott
    Apr 29 '15 at 17:02
  • $\begingroup$ james large, Rightly said , optimal for fuel economy. $\endgroup$
    – NG_21
    Apr 30 '15 at 5:52
  • $\begingroup$ @jhabbott I believe that's what we call "rejecting the premise of the question." $\endgroup$ Apr 30 '15 at 21:09
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In a modern car with electronic engine control, fuel injection, etc, the amount of engine power is always dependent on what the ECU is doing in response to the driver's control inputs like throttle position, brake pressure, etc. If the ECU also knows what gear you are in, it is straightforward to calculate whether you could produce the same amount of power more efficiently (i.e. with lower fuel burn) in a different gear. Automatic transmission cars have been doing that "calculation" for decades (originally with a simple mechanical "computer" instead of electronics), to decide when to shift gear.

The big thing that is missing from the systems I'm seen is the lack of situational awareness of what is going on outside the car, which is a severe limitation on the usefulness of the systems. You probably already know that following a line of traffic at a steady 30 mph on a level road is more fuel-efficient in 5th gear than in 2nd or 3rd, but what you can see through the windows might be telling you that 2nd or 3rd is a much better choice in the real world situation.

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  • $\begingroup$ Thanks and rightly said, the contextual awareness is not there. Any web resources or literature which I can read about the algorithm of ECU would be very helpful $\endgroup$
    – NG_21
    Apr 30 '15 at 5:55

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