Effect of Speed on Fuel Consumption /Mileage

Effect of Speed on Fuel Consumption /Mileage


Fuel sapping at its worst

We all know that during bumper to bumper traffic, fuel consumption will suffer. How does this happen?

A car engine, or engineering wise, an internal combustion engine is only efficient in a specific range of engine speed, or revolutions per minute. It’s maximum efficiency occurs in mid rpms.
At low engine speed, an engine cannot burn fuel effectively, thereby reducing fuel effectiveness. More over, usually during low rpm, or worse, idling, a vehicle does not move much, or barely moves at all, especially during traffic jam. This certainly results in low mileage.

Albeit differently, an internal combustion engine does not work as efficient either at very high rpm. It’s due to the high engine speed which does not give enough time for a good combustion cycle, thus does not allow fuel to burn satisfactorily. This, in turn, also contributes to a lower effectiveness in fuel usage.


Best range efficiency for the engine

During mid rpm, an internal combustion engine reaches its best thermal efficiency, thus it gives best fuel efficiency at this engine speed. And usually a car driven at mid rpm usually runs at a constant, relatively high speed (at the highest gear).
Nevertheless, running a car at too high speed which induces high engine rpm may lower mileage, due to the aforementioned reason.

Thereby, selecting appropriate speed, in consideration of safety, may lead to a better fuel mileage.

The two following pictures show mileage (fuel consumption) for a specific car for two different vehicle average speed.
(pictures). The pictures were from 2 subsequent trips from approximately similar flat road terrains, thus no effect of climbing /rolling downhill is considered.

speed 18
mileage at 18 kph average
speed 27
mileage at 27 kph average speed

It is evident that the higher speed leads to better fuel economy. Be cautious though, that increasing speed further beyond efficient engine speed will also increase fuel consumption.

So we can improve fuel economy /mileage by aiming for a speed where the engine works at peak efficiency, that is, at mid rpm. Be warned though, that safety is the utmost concern during driving.


A key to efficient fuel usage

Here is the key: aim at driving the car at mid engine rpm at the highest gear possible.
I will post details and tips on fuel saving (and of corse, on family fun car living) articles sometime later.

By the way, the measurement was made with Scangauge II, installed on a Toyota Avanza (Toyota F651 car model), Indonesian version, with K3-VE 1.3 liter engine mated to its standard AQ4 Aisin automatic.