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
The resistance of a car moving at low speeds is dominated by rolling resistance.
At higher speeds, you still have the rolling resistance, but the resistance is dominated by wind resistance, which increases with the square of the speed. Thus, the wind resistance at 50m/s is not 10 times greater than 5m/s, but rather 100 times greater.
The main thing is drag. When you double your speed (with everything else being equal) you quadrupole your drag. You also quadruple your energy (which is part of why it takes much longer to stop from 60mph than it does 80 mph). Thus the drag your engine has to overcome increases much faster than your speed increases. At lower speeds it doesn’t matter much. At highway speeds it will be something noticeable on your fuel economy.
Your school theory is perfect in a vacuum and in an environment with out things like drive train losses. If you are on a flat surface, the main thing that slows down your car when its not powered is wind resistance, and then rolling resistance. Rolling is somewhat minor in comparison. THis is why when you left off the gas at 80mph without the brakes, you slow down pretty quickly, but you can roll for a long time from 10mph before you stop (because the wind resistance is very low at 10mph compared to 80mph)
Imagine a spaceship that’s going to the opposite end of the galaxy. It has a massive pulse engine that uses as much power in a second as our sun outputs in a day, to accellerate that ship up to almost the speed of light. It coasts through the galaxy using no power, and when it arrives at its destination it again pulses its massive engine outputting as much power as the sun.
Now imagine it made the same trip, but to the local grocery store. Aside from completely incinerating the grocery store it’s going to (and the rest of the planet Earth), it would use the same energy as for it’s intergalactic trip. Btu we can obviously travel the same distance for less energy if we just go slower.
Really it’s the exact same principle in a car.
energy loses in environment without being used by the engine when you pump up your engine with gas
Combustion engine cars are tuned for peak efficiency at around 80kph (50mph). Internal combustion engines actually have the best fuel in to power out ratio at around 70% load believe it or not. So if you drive too slowly and never load up that engine it is counter productive. Cars with smaller engines get better mileage as the engine spends more time ‘heavily loaded’.
Step up to the bigger engine and you aren’t loading it enough for peak efficiency. But if you are often driving very fast, climbing mountain highways or towing that bigger engine isn’t being overloaded as often so it may be a better choice to you.
Above around 80kph (highly dependent on aero), drag takes over and load increases exponentially.
Electric cars ARE more efficient at slow speed.
Hybrid cars allow that engine to be downsized so it can spend more time in that peak efficient loading, coasting or not running at all. The electric motor takes up the slack.
YES – for several reasons.
**You are using energy to accelerate your car up to some speed.**
The force you need is linked to both the mass of your vehicle and the acceleration. The equation from school physics F=M*A (force = mas X acceleration) suggests that the force required is proportional to the acceleration used to get it from 0 to whatever speed you cruise at.
Now the bad news of inefficiency, wind resistance and friction.
* wind resistance – your car has a shape that pushes air out of the way. This also takes energy. The faster you go the worse this gets (exponentially I believe)
* Inefficiency comes in so many places.
Your wheel surface is moving in the wrong direction 75% of the time (up, down, forward) and this uses more energy at higher speeds.
Tires change shape (flatter on bottom) and this takes energy as you drive.
Transmission is spinning all the time
Engine is spinning all the time
Most cars have a “sweet spot” for best fuel economy. Everything different is worse.
I think air resistance is the biggest factor. Air resistance goes with the square of the velocity. Accordingly, if you double your speed, air resistance increases by a factor of 4. Triple it and it goes up by a factor of 9.
1. Wind resistance. Faster speed = more wind resistance. Stick your hand outside a car window while driving and feel the difference between slow and fast speeds. That is whats “pushing” against the car while driving.
2. Modern cars have transmissions (The thing that converts engine power to wheel power), and are optimized for specific speeds. Low RPM but high gear is more efficient than high RPM and low gear.
To expand on the “why does air resistance increase with the square of speed, and not just with speed?”
Intuitively, you can think of it like this: when you double the speed, you have to double the force your car is pushing the air out of the way. But on top of that, you are also pushing ***twice*** the air out of the way (because you are going twice as fast).
So one factor of speed comes from the speed you have to push the air and the second factor of speed comes from the amount of air you have to push. Two factor of speed is just speed squared, exactly as claimed.
The harder you accelerate, the worse your fuel consumption is. The faster you drive, the harder the wind resistance holding you back is. Every bit of braking you do is wasting energy you used accelerating in the first place.
So, accelerate slowly, maintain low speed, coast early instead of brake. You see truckers do this all the time, they slog up a hill very slowly, coast half a mile to get the green light rather than stop and start up again, and draft behind other trucks so they fight the wind resistance instead.
Latest Answers