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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?

Pᴀᴜʟsᴛᴇʀ2
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Palak.Maheria
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3 Answers3

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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.

Shobin P
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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 Jul 21 '15 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 Jul 21 '15 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 Jul 21 '15 at 13:40
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    @Paulster2 I know, that is why i said, One of the "SMALL" reasons. – Shobin P Jul 21 '15 at 13:42
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    Haha! Great point and observation :D – Pᴀᴜʟsᴛᴇʀ2 Jul 21 '15 at 13:44
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    How did you know the OP's car has a 1.2L engine?? – Jongosi Jul 21 '15 at 16:22
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    @Jongosi research my friend :-) http://mechanics.stackexchange.com/questions/18495/i-have-had-wheel-alignment-and-balancing-several-times-still-my-car-is-shaking – Shobin P Jul 21 '15 at 16:45
  • My car is Fiat Punto 1.2 petrol – Palak.Maheria Jul 22 '15 at 04:47
  • @Paulster2 but gasoline engines generate peak torque at around 4000rpm and isnt it the rpm range where more fuel is consumed? – saurabh64 Jul 22 '15 at 06:22
  • @saurabh64 - Actually they don't ... it's all dependent upon air flow, and that depends upon heads, cam, and exhaust as to where that occurs. Usually torque peaks early and fades slowly as rpm increases. This dyno graph of a VW 1.8T engine is fairly typical of what you'd see in today's engines. Peak torque comes in ~2500 rpm. Anyway, not a "standard" but fairly typical. – Pᴀᴜʟsᴛᴇʀ2 Jul 22 '15 at 14:56
  • @Pᴀᴜʟsᴛᴇʀ2: At max torque, fuel efficiency is best, not milage. That is, you get most mechanical power out of a certain amount of fuel. But if you don't need max power, lower RPM is better. Fuel efficiency might be lower, but since there are less bangs per second, the overall fuel consumption is lower. So, if you want to accelerate, use max torque RPM. If power demand is low, use a low RPM, where the motor delivers the power easily. – sweber Feb 04 '18 at 16:54
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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.

David Xu
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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.

Rodney
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