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I drive my car a lot. I do lot of experiments with speeds while driving. Following are my observations:

  1. Driving at 60 - 70 km/h gives maximum mileage
  2. Driving at 70 - 90 km/h gives good mileage
  3. Driving above 90 km/h gives poor mileage

All the above observations are measured while driving in top gear (5th gear).

I can understand driving at 60 we get maximum mileage but there is not much difference in mileage while driving between 70 - 90 km/h why is it so, can anybody explain this?

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Mileage of a car is based on a ton of factors but since you are concerned with a specific speed range it comes down to one thing Air resistance.

  • Air resistance: There is very minimal difference in the air viscosity between 70 to 90 kmph but when you go more than 110 the air starts to get thick , simply put , if your car's aerodynamics are optimum to handle a particular speed then it will be more efficient at that range.

Also since your car is a 1.2 it will be happy to cruise at those speeds , always remember a big engine working at smaller RPMs will be much more economical when compared to a smaller engine working at maximum load.

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    I don't have a reference for this, but I understand a vehicle gets its best fuel mileage at peak torque. I think this backs up what you are saying about working a small engine hard v. a big engine easy. Peak torque usually happens in the lower rpm range on any engine.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 21, 2015 at 13:01
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    @Paulster2 Exactly .. That is one of the small reasons why diesel engines are more efficient, they produce peak torque at very low rpm.
    – Shobin P
    Commented Jul 21, 2015 at 13:07
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    Not exactly why. Diesel has a higher energy content than gasoline (petrol) as well as diesel engines utilize more of the heat energy than does gas. These are the main reasons why they get better fuel mileage.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 21, 2015 at 13:40
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    @Paulster2 I know, that is why i said, One of the "SMALL" reasons.
    – Shobin P
    Commented Jul 21, 2015 at 13:42
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    Haha! Great point and observation :D
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 21, 2015 at 13:44
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I'm pretty sure it is not due to aerodynamics. It has got to do with designed optimal engine rpm. Your 1.2L car is not really designed to run best at highway speed.

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My experience is that the fuel consumption curve in my vehicle is hyperbolic. In the low speed ranges as speed increases fuel consumption doesn't increase by much. But it gets progressively worse as speed increases. At 120 kph the curve is approaching vertical (on the scale I used). I presume the explanation is wind resistance.

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