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My car is an Audi S5 which has the normal cruise control (non-adaptive) with auto braking (applies brakes when going downhill too fast).

For some reason when engine braking the car uses around 4L/100KM instantaneous petrol. If I disengage cruise control, it will use 0L/100KM as it shuts off fuel. Therefore, I prefer to turn off cruise control sometimes when going down long roads.

Is cruise control just an electronic software, or are there mechanical / mechatronic devices which get engaged/disengaged when you turn CC on/off? Is there any harm in turning CC on/off a lot of times?

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The cruise control on a modern car is totally electronic – obviously it has to interact with the car's controls, but no more so than would happen in "manual" driving. There should be no reason why you can't turn it on or off at will.

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I have VW Golf, which technologically should be similar to Audi, and I observe the same behavior you describe. I think the reason to keep the fuel on is that ECU can keep the speed constant in more cases, than with fuel shut off. Also with cruise control it seems to consume more fuel when you have many uphill/downhills compared if you use throttle manually.

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    I find going uphill, the CC does a better job than me, and going downhill I do a better job than CC.. (fuel consumption wise). – tgun926 Dec 29 '16 at 23:23
  • Maybe because going uphill CC keeps speed constant, hence less fuel consumption (all other things equal), but going mixed up and down the road, you can accelerate a little bit before uphills and decelerate before downhills, hence getting less fuel consumption overall. – oryades Dec 30 '16 at 11:02

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