Hi,
For a calculation of the mpg (miles / gallon), means that there are two parameters to be measured, miles and gallons.
First the miles :
Miles is measured by the odometer, and is coupled to the speedometer. The speedometer is coupled to a cable, and with a few gears it's coupled to the wheel. The wheel can be seen as a gear and the opposite gear is the road itself.
It seems simple, but when using another tyre, your reduction ratio has changed, so does you measured speed, and so does your odometer. So I have determined that the distance measurement in itself is relative, and can hardly being compared to other bikes.
The best way to determine the distance is to measure it absolute, but a GPS is very practical too and accurate enough.
Then the gallons :
Let's assume that the petrol station is well calibrated, because if it isn't, one should measure the volume yourself.
Patrol is not only highly flammable, but also vaporizing quickly, especially the lighter molecules. The fast vaporizing molecules are also the highest energetic molecules, so it's important to use fresh gas for the test.
It is impossible to measure the volume of petrol by eye in one measurement in the gas tank. You never know if the bike is in the same position (straight, angled etc). The best way to measure is to do it more often, and determine an average over let’s say 10 times filling you gas tank with at least more than 75 % of the gas tank volume. This will reduce the influence of mistakes. Make sure the gas is used by the engine, and not leaking out of bad tubes or petcocks.
If the above is done, then the calculation is precisely enough isn’t it ?
But the engine isn’t running on gas only, it runs on a mixture of air and gas. Let’s exclude all the parameters of the carbs and engine itself, there’s still a factor of importance, and that the mass of the air.
The fuel / air ratio is a mass ratio. Since the mass of the gas can be seen as fixed, the mass of the air is not. The mass of a gallon air differs with temperature and height, and with the amount of moisture. So the fuel mass ration is depending on temperature, height and moisture.
As one sees the engine as a mixture pump, the amount of mixture is determined by the pump efficiency and the rpm rate.
Let’s say that the pump efficiency is to be measured, then the rpm is an important factor to keep in mind while comparing measurements.
And hence using the choke, short distance riding, long distance riding, riding in winter, summer, autumn, or spring, sea level or mountains.
But the pump efficiency isn’t to be measured, it’s the amount of energy per stroke per cylinder that determines the output of the crank, and the rear wheel. There are four cylinders, so the efficiency of one cylinder can be destroyed by its neighbor (so it’s important to synchronize everything). Then there’s wind, internal friction (for example bearings), the oil you used the weight of the rider and many, many, more.
So before concluding anything on mpg, be aware that these factors are counting.
So, is it impossible to determine a good and reliable mpg and compare these figures between people with the same bikes ?
No, as long as these figures are interpreted correctly, and that means that differences in mpg between two equal bikes can be caused by many factors, and most of them are not related to combustion efficiency.
I'm a physics engineer, so I’m imperialized to see things in a simple way (can’t help it).
Jensen
