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 advertised number is based on a specific test condition
The EPA has two profiles, one for city and one for highway, that involve speeding up, coasting for a bit, slowing down, speeding up, etc over the test. The city test is about 20 minutes long and the highway one is 12.5 minutes long. The rated fuel economy is only true for those specific test profiles which is why people generally see different real world numbers
You can see the test profiles here
https://www.fueleconomy.gov/feg/fe_test_schedules.shtml
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.
From my knowledge it is based off speed, there’s an old chart where at lower speeds you burn more fuel the faster you accelerate then it reaches a point where you stay at a constant speed and and fuel consumption kinda levels off to be an efficient value. In the states its normally based of 60 mph and 35 to set for highway and city driving. City driving is stop and go so, its alot of mashing the gas to speed up then hit the brake to slow down, where as highway you hit the gas get to speed and stay there til you have to stop or something causes you to slow down but it not as stop and go as in a city. Example turn on a faucet just a little, the water comes out at a slow rate, example of reaching a good fuel burn to movement ratio at a constant speed, if you turn the faucet wide open (hard acceleration) more water comes out similar to how you would burn more fuel. Sure someone can give a better example with demos and pictures
Think about it like a bike on a flat road. When you start you’re probably on a gear that translates a lot of leg movement into not a lot of bike movement. As you get moving you’ll shift gears so you can increase bike speed without increasing your leg speed. As you progress you won’t really be working harder, more like just maintaining and slightly building on the momentum you’ve established. At a certain point, you’ll run out of gears and to go faster you’ll have to start pedaling faster expending more energy to increase your speed.
Cars are similar in that they shift gears to keep the engine spinning at a certain speed up until they’re in that top gear where the RPMs are low and speed is high. Then if you go faster increasing your RPMs without another gear to shift into, you’ll go faster but burn more fuel (and instead of building muscle, you’ll wear your engine out faster).
As a supplement to the other answers, fuel efficiency goes down as vehicles reach high speeds because they have to push harder through the air. Theoretically the force goes up with the square of speed, so a car that gets 40mpg at 50mph should get around 10mpg at 100mph. Engine efficiency and other sources of friction mean the difference won’t be so extreme.
Note that the metric unit for fuel economy is litres per hundred kilometres and low numbers mean better economy.
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