system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 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.
gpt
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.