How is it much more fuel efficient to travel at say 50 mph for longer than 70 mph for a shorter period of time. Like if I was going a set distance, if I drive slower I’m using the car longer so why isn’t it the same fuel being used.
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In short: It’s sometimes not. What a lot of people don’t take into account is today’s transmissions making the engine extremely efficient at creating movement at the wheels, but aerodynamics allows cars to be far more efficient than they once were.
Example: Take a Corvette made anytime in the last 20 years or so. The gearing is usually designed so that the engine only has to turn at say 2,300 rotations per minute in order to make the car maintain roughly 80 mph in its top gear. Those 2,300 rpm would only allow the car to go maybe 25mph or so in the first gear. The transmission by itself allows the car to use relatively little fuel to maintain 80mph once it gets there. In Corvettes in particular, tests have shown (due to transmission as well as their massive engine) their most fuel-efficient speed is actually around 85mph. Each car has a certain speed where its engine size, transmission, and aerodynamics all create a “peak efficient speed,” and a few decades ago, that speed was closer to 55mph.
Long Story Short: Engines use less energy due to having transmission gears, but this is usually only up to a certain speed before wind resistance overcomes it, and it takes far more power to go even a little faster. Sportscars are not only designed to be pretty, but they’re also very nearly aerodynamically perfect, meaning they get way less air resistance so they cut through the air better.
To add to what others have said, not only does air resistance scale exponentially with speed, but also engines have an optimal RPM efficiency, generally they’re the most efficient when the car is in the highest gear (giving the most mechanical advantage, and most distance traveled per engine revolution) and the lowest (or on the lower end) RPM to maintain the torque to stay in that gear. Balancing all of this, most cars are at peak efficiency when traveling at 50-60mph because they can be in the highest gear, and going faster would have much more air resistance.
One rather forgotten factor also tends to do with engine efficiency. An engine running at a higher load (for the same RPM) is more efficient.
Eli5 version.
Walk a block. Takes some energy not a lot.
Now run a block. Goes quicker, but you’re breathing harder right?
It’s like that.
If you had to push a fridge 50 feet, which is easiest? Push it at a snails pace but never allow it to stop, push it at a slow walking pace, or push it at a sprint? The slow walking pace is easiest, just because it’s natural and fits your bodies biomechanics. The engine has a preferred speed too, and most manufacturers make it so that it’s preferred speed is roughly highway speed.
Air resistance is quadratic with respect to the speed you’re traveling. So the amount of force air is pushing on your car at 20mph is quadrupled at 40mph, etc.
Time saved from going faster is only linear. 40mph is twice as fast as 20mph.
So you’re increasing the amount of resistance your car has to overcome quadratically while only reducing your travel time linearly.
You measure off the unit which is constant, meaning the distance not the time. If you were measuring off the time you would be using more fuel per minute.
If you travel 100 miles at 50mph it would take you 120 minutes. You would maybe do that journey at 50miles per gallon or 60 minutes per gallon.
If you were to travel at 100 miles at 100mph, just for the ease of maths and understanding, you would complete the journey in 60 minutes. You would probably complete that journey at 20miles per gallon or 12 minutes per gallon.
You have done the journey in half the time but you have used 250% the amount of fuel for the same journey.
It is mostly due to wind resistance. If wind resistance were linear, then going faster for a shorter period would be just as efficient as going slower for longer. But wind resistance actually increases with at least the square of speed, so that going faster uses more energy over the same distance than going slower.
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