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We have two same cars,same weights,just with two petrol engines, one has 200hp and second has 400hp.engines have same efficiency factor.

In theory at same constant speed at highway of 140km/h cars have same drag so power delivery from engines must be also the same.

Will fuel consumption be the same for both engines at that constant speed?If not way?

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I believe they would have the same fuel consumption if the only difference is the rate of change of energy (also known as power). The change in power between the cars is only the ability to produce more energy in a shorter time period. Since they are travelling at constant speed and they have "same efficiency factor" (key term to the question) which I guess you mean the energy conversion from fuel to mechanical work is the same for the both cars. And since they both travel at constant speed, then they will have the same fuel consumption. The one with more hp will however accelerate faster.

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  • $\begingroup$ But what if they have peak efficiency at different RPM? $\endgroup$
    – 22flower
    Commented Oct 10, 2021 at 11:22
  • $\begingroup$ Engineering stackexchange can probably give a more educated answer to this since the question is leaning more towards car engineering than pure physics. My guess is that if they do not share the same peak efficiency at same RPM than the original assumption of the cars having the same efficiency factor cannot apply. Higher RPM usually means higher fuel consumption. So the car with higher peak efficiency RPM will consume more fuel than a car which is exactly at is peak efficiency when going at some constant speed. $\endgroup$
    – ludz
    Commented Oct 10, 2021 at 13:19

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