Does this advice apply to all cars?
In: 156
Accelerating – Higher RPMs consumes more gas, keeping RPMs lower consumes less gas. Engines are not 100% efficient, so some of the energy produced is wasted as heat or to drive other systems in the vehicle (Such as AC, water pump, or cooling system). Higher RPMs are also associated with traveling faster in general, so more wind resistance.
Braking – Also ties into engine RPMs, as your engine is still running, if you brake suddenly, you are likely to have been running at a average higher RPM for maintaining speed instead of a lower RPM as you brake gently or “coast” to a stop.
For electric vehicles, there is also regenerative braking.
Braking = getting rid of energy, aka gas. Coast more, brake less, save gas.
Hard acceleration = high engine RPM. Slower spinning engine means less energy lost to friction.
Both of these in moderation. Most of the energy saved in coasting is at high speed. 60 -> 40mph is worth more than 40 -> 0 mph. Accelerate too slowly, and your car spends extra time in lower gears where it has more engine speed.
It’s worth noting that even with hybrids and battery-powered EVs you will generally get better efficiency if you brake gradually and accelerate slowly.
When you brake in one of these cars it will use some of the energy to charge the battery back up, and some of it gets lost as heat. The harder you brake the more energy gets lost as heat.
Similarly, the faster you accelerate the more power is needed. In a hybrid this might cause the gas engine to kick in earlier and rev higher, which reduces efficiency. In a battery-powered EV more power means more current, which means more energy wasted as heat.
Battery-powered EVs don’t lose as much efficiency when accelerating quickly, but there is still *some* impact.
As to your second question: No. Some cars are designed amazingly well to go really really fast really really quickly and some actual regain mileage from braking just right and not gradually (EV or hybrid).
As to your first question:
Inertia.
The ELi5 is that mass doesn’t want to change what it’s doing.
If it’s at rest it doesn’t want to start moving.
If it’s moving it doesn’t want to stop.
It’s reasonably instinctual when you just push things around but you can very easily measure it.
Without going into the math inertia fights you harder the more change you want to impart.
Pushing a marble to go 1 kph in 1 sec across a table takes a tenny tiny amount of energy.
pushing that marble to go 100kph across a table takes more then 100 times the same energy.
In the same way a baseball thrown at you at 10 kph will hurt but one at 20kph will hurt more than twice as much.
If you think about accelerating a car a series of small steps for each kph then using the previous analogy if you make the steps longer inertia doesn’t fight you as hard for every step and you use less energy.
With braking most cars actually don’t car BUT if you time it just right you can roll until the light turns green and you can then start moving again without coming to a stop. That saves you a bit of energy since you don’t fight inertia from 0 kph.
Braking doesn’t DIRECTLY save you gasoline. However the more you need to use the brakes, the less fuel efficiently you are driving overall. The brakes of your car convert. EXTRA forward momentum you didn’t need to get where you’re going (kinetic energy) into wasted heat energy as friction on your brake pads (which also wears them out faster, btw). That excess momentum exists because you used your gas pedal to accellerate more than necessary. The more you need to brake, the more it indictates you used your more gas than you needed to get your car to the next stop (whether its the next red light, a yield-turn, your end destination, doesn’t matter which).
The more the car accelerates the more it consumes energy, which in turn, consumes more gasoline. Breaking gradually doesn’t necessarily give better mileage (unless you drive an EV) but you usually do it before a stop sign because in case the light turns green, accelerating from slow speed consumes less energy than accelerating from complete stop.
I would think of accelerating slowly like the human body going from stop to walk to jog to run over going from stop to sprint.
When you press the gas pedal, the car drinks fuel, if you press it further. It drinks even more fuel, so to minimize the fuel consumption you should press the gas pedal as less as posible, and for accomplish that, it’s a good idea not to press the breaks pedal neither.
Basically everytime you touch the gas pedal you are turning gasoline into kinetic energy (and heat because thermodynamics always wins), and every time you touch the brake you are turning kinetic energy into heat. The more time you coast without touching either pedal the more distance you travel for a given amount of kinetic energy.
Transmissions are designed to shift based on demand. If you hit the gas hard, the transmission will shift later. Shifting later means higher rpm. Higher rpm, means more fuel burned (also getting to that rpm faster, means more fuel burned) …. Basically your car thinks “I need maximum power, right NOW!”, and it revs higher and faster to get this done, resulting in a lot more fuel being dumped into the engine to accomplish this… Over time, this adds up quite a bit.
Same phenomena happens if you change to manual shifting in an automatic. The car is optimized to shift efficiently… Shifting manually means you likely shift at higher revs , and you will burn more gas. I noticed this quite a bit, when I had my 350z… Enough that I stopped driving it in manual mode for everyday driving.
As far as your braking question… Only thing I can think of, is that you let your foot off the gas, so the car essentially ‘limps’ with low engine revs…?! Bevause there’s no “demand” from the accelerator. Vs braking harder means the revs were likely higher for longer… But this should be negligible in the scheme of things.
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