In school we learned ” what you save in energy, you have to increase the way.” By that rule you should use the exact same amount of energy (fuel) for the same distance no matter what speed. I’ve asked a few people, but no-one could give me a good answer.
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You need a greater amount of energy to reach and maintain a higher speed, because you have to overcome more air resistance and it’s not linear. Let’s say you need X amount of energy to keep a car at 100 mph. If you increase the speed to 200 mph, the amount will increase to something above 2X, and so on.
It’s not only about air resistance, the reason nothing with mass can reach lightspeed is because it would require infinite energy, but this is ELI5
Two reasons –
1. Car engines do not have the same efficiency (likes gallons of gas per miles) at all speeds. In reality engine efficiency is mind bogglingly complex but manufactures “tune” the engines they design to *most efficient* around the speed limit-ish. So outside of any other reasons, a car engine might get 20 miles per gallon driving at 50 miles and hour, 30 miles per gallon driving at 60 miles per hour and 10 miles per gallon driving at 70 miles per hour.
2. Air resistance increases exponentially with speed. Meaning there is much, much more air resistance trying to push back on a car when it drives at 70 miles per hour than there is when driving at 60. So it takes more energy (burns more fuel) to keep a car driving at 70 than it does at 60 because the car needs to overcome that air-push-back.
EDIT – to clarify what you learning in school – you’re talking perfect, ideal physics. Yes, you can do physics math to calculate the energy required to move a car from X to Y. But that’s the ideal, perfect, *minimum* energy required. In reality something like a car engine can only extract around 30% of the chemical energy in gasoline and then turning into motion energy in the engine with all the moving parts sucks that down, and air resistance sucks that down, friction between tires and the road sucks that down etc. etc. So when we talk about real life systems the method of getting from X to Y is very important.
Two reasons.
Engines are designed to have an efficiency “sweet spot,” I believe around 55 mph.
Also, drag from the air is not linear with speed. If you double your speed, you more than double the air resistance. The same is also probably true for other friction losses.
Take a distance, perhaps from your garage door or house door to the corner, let’s say its 100 meters.
Walk the distance and back.
How do you feel?
Now run at a slow pace.
How do you feel?
Nor run like your life depends on it.
How do you feel?
You went the same distance in the three scenarios, but the feeling afterwards it’s completely different.
Your body had to use more energy to travel the same distance, because you were demanding more power/speed.
A car engine is no different, you can cruise at 50mph with an engine that’s not working hard, and will require less fuel, or you can cruise at 90mph demanding more power and making your engine thirstier thus consuming more fuel.
I’m oversimplifying, but this is how I understand it, also in manual transmission cars, it’s easier to make the engine work at lower revolutions by using higher gears. (This is, or was, a common trick Taxi drivers used to save gas and improve engine life).
There are 3 major sources of fuel consumption
Baseline – this is pretty constant and is the fuel needed just to keep the engine spinning at any speed
Rolling resistance – this is from the tires squishing. It scales linearly with speed so 40 mph requires twice as much power to overcome this as 20 mph but you cover twice as much distance so it washes out
Air resistance – this scales with the square of speed so 80 mph takes 4x more power to over come air resistance than 40 mph
The end result is that a plot of speed vs fuel use per mile looks like a bowl with a low spot somewhere in the center and higher consumption at really low and really high speeds.
At low speeds, the fuel required to keep your engine spinning dominates and you aren’t covering much distance so the result is pretty terrible.
At high speeds air resistance dominates all else and going from 100 mph to 110 mph will increase fuel consumption by at least 21% despite only increasing speed 10%.
The range in the middle is where it gets tricky and depends on the car. Modern cars have lower rolling resistance tires so item 2 above is minimized, they’re also far more aerodynamic which reduces air resistance and changes the slope of the curve. They’re generally pretty good in the 40-75 mph range these days, partly thanks to the increase in gears(3 speed autos have been replaced by 9 speeds) so manufacturers don’t have to pick just a single upper speed that is in the engine’s happy range.
In order to move something you must continuously supply it with energy. In the case of our cars we use the burning of gasoline to turn the wheels. The faster we go the more energy we lose to friction, mostly air resistance. Therefore to maintain that speed we need to consume even more gas per mile than we would if we went slower and had less air resistance.
Some good answers already, but one thing I think is worth mentioning:
Imagine that you have to do a 1-hour drive, for example, with traffic lights and crosswalks every few minutes. Each one of those ‘obstacles’ in the road will make you slow down to a reasonable driving speed, so that you can check for pedestrians or even fully stop for a red light. After every time that you slow down, you want to build up back to the speed you were driving at, basically the speed you want to cruise in. If you want to reach 40 MPH, you have to feed the engine with an increased amount of gas to get to that speed, and then feed it a regular amount to stay in that speed. If you want to reach 60 MPH you would have to feed it an increased amount for a longer period of time after every obstacle and slow-down. Also, at that increased speed the amount you would have to constantly feed it just to keep driving at that speed is increased, as other people have mentioned, due to air resistance, mostly.
That is why sometimes people say that using the brakes wastes gas – it’s not that it takes gas to use the brakes to slow down, it’s that you already put in gas to built up that speed, and braking basically gives up that speed. Idealy, you would want to drive at speeds that are optimized for the road you’re on, if there are a lot of things causing you to slow down and speed up, in order to use the breaks as little as possible – that way you know you’re using the amount of gas you actually need.
If your engine uses less gasoline per mile at a slower speed, then it is better to go at the slower speed. Engines are made to get their best fuel milage at about 35 mph (72 kph) and 55 mph (88 kph).
That’s just where your engine will be the most efficient. Going faster decreases your efficiency. There is a sweet spot where your engine and transmission work less to combat the air it has to push against.
Let’s say you were going down a road at 55 mph getting around 35 mpg. The same car going down the road at 60 mph is only getting 30 mpg. The second car would only be about 5.5 seconds ahead after a mile. If you were traveling 6 miles, that would only be about a minute behind. Speeding doesn’t really get you there very much faster unless you are traveling over 100 miles.
Are you willing to buy more gasoline every year to save yourself 2 minutes a day? It’s a personal question and some would say yes; not me, but some would.
The explanations that others are giving are all correct, but I’d like to add one small detail, drag and inefficiencies are the most important factors but the kinetic energy of the car is also important:
The kinetic energy scales with the square of the velocity, while the distance traveled is directly proportional to the speed, and every time you brake and accelerate again you have to give spend that energy again.
The rules of physics are generally applied in a vacuum. So theoretically once initial resistance is overcome the energy required to move something remains constant despite changes in rate.
But when we are discussing everyday transportation examples we are not only moving an object, but also constantly encountering friction and displacing air. Each of those episodes requires the moving object to change the state of the other things it encounters and that is where the additional energy is used. Not to move the mass itself but to overcome outside forces. And the faster an object moves the more outside influences it encounters in a given period of time. Hence more energy expended/ unit of time as speeds increase.
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