why do you save fuel if you drive a distance slower.


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.

In: 15

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.

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

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).

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.

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.