In what way does the mpg or kmpl number of a car depend on its speed? Is the advertised number based on average speed or a specific speed (say, 60 km/h)?
In: Engineering
The calculation itself isn’t related to speed. Speed comes into play relative to the work the vehicle is doing. Generally, higher speeds mean that the drive train has to rotate faster to spin the wheels faster. This is done by burning more fuel to increase the force and number of explosions in each engine cylinder.
Some efficiency is saved by having multi-speed transmissions. As the engine achieves higher RPMs, the transmission switches to a larger gear, which turns the wheels more from in single rotation. This means that the engine doesn’t have to work as hard to move the car at faster speeds.
Physics also affects it. Because of inertia (in short, an objects tendency to resist change) and friction with the road, it takes more energy to get a car up to speed from a stop than it does to keep it moving at a consistent speed. This is why cars universally get better gas mileage on the high ways (consistent speeds) than in towns or cities (starting and stopping motion).
I think that covers about all of it. Let me know if you’re still confused and I’ll try to clear things up more.
Edit: Forgot one part. Most vehicles are designed for highest efficiency when traveling at 55-65 mph, so depending on how many gears your transmission has, you can start losing efficiency the further you go past that range.
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