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Comment by rtkwe | Hacker News Reader

Highway miles are generally the most efficient miles for a vehicle because there's less energy wasted in slowing and speed changes, at least for gasoline cars though I imagine the same factors are present in electric vehicles minus maybe the braking losses since they can use regenerative braking. Is that alone enough to make city more efficient for electric vehicles?

edit: Just judging from my own observations (and the EPA estimated differences) I get way better mpg when I take long drives on the highway cruising at 65-70 than I do on periods with mostly city driving. I keep track when I fill up as a check against the car's slightly wonky mpg calculation and it's a difference of 5ish mpg more for highway like the EPA estimates suggest.

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For EV cars(1), air drag is the highest range killer. There's a huge difference in range depending if you go 65, 70 or 80. And if you follow cars or push the air by yourself.

Stop and go doesn't really impact range that much, and the low overall speed is a better regime. Although these cars accelerate like crazy so it's hard to resist the pedal when doing stop-and-go.

(1) in fact for all cars, but ICE cars are so inefficient in other regimes that air drag is not a prominent problem until much higher speeds.

Not the case for electric. Batteries are heavy, and it takes more current to maintain a faster speed than a slower speed. Plus as you point out you don't get as much regenerative breaking.

You can see this in regular gas hybrids that get better city mileage than highway mileage. The effect is even more pronounced in full electric vehicles.

The weight of the battery won’t affect the energy needed to keep a car at a steady highway speed. The energy needed is just whatever losses come from friction, air resistance, etc

The weight of the battery is related to the amount of friction.

At 70mph most energy is lost to wind resistance.

EVs are also heavier, which increases rolling resistance on the tires. This lead to the formulation of stiffer tires for EVs to counteract that effect.

> Plus as you point out you don't get as much regenerative breaking

Regenerative breaking is not a benefit compared to steady driving, it's a waste of energy due to the second law of thermodynamics.

It's only a benefit when you compare it to non-regenerative breaking.

Yes. When I had a Nissan Leaf for a couple of months, I couldn't make it to the office and back without a partial recharge on the way home. But I discovered if I got off at an exit fifteen miles earlier than normal and took surface streets, I could make it home with a bit of capacity to spare. That's not something that would have been good for the battery if I did it every day but at the time I was going into the office about once a week so it worked out fine. Plus I was using the 110V charger at home, which would take more than the next morning to charge the battery fully.

The biggest loss at highway speeds is actually wind. I think wind resistance goes up with the cube of speed.

As a result, electric cars are more efficient in typical city driving. You can see it in the MPG stickers , which are higher in city than highway for EVs

For gas cars, their most efficient speed is typically a steady 35-40 mph. For an electric, it’s 15-20. If there were no wind resistance both these numbers would be substantially higher.

No, the best MPG is at 30-40 mph with ICE vehicles due to auxiliary loads and losses at lower speeds and wind at higher.

I would guess BEVs have their breaking point at even lower due to lower idle power losses.

If you can maintain constant speeds it would be but the only places I drive in that range is in cities where I'm stopping and starting a lot so the wasted energy there dominates. I keep rough track of my mpg each time I fill up (as a sanity check to the calculated MPG my car gives me) and when I take long trips home I easily hit 33-34 mpg while weeks where its mostly short trips in the city I usually get more around 28 or lower. And those long trips sit at 60-70 for an hour or more at a time in good traffic.

Ye that is correct.

My commute is about 10 miles of which 8 is 40 mph two lane country road. So I think I got about the lowest mpg my car can have.

Once you hit 65mph and above your gas mileage and electric wh/mi drop like a rock due to air resistance scaling non-linearly.

I get way better mpg when I take long drives on the highway cruising at 65-70 than I do on periods with mostly city driving. I keep track when I fill up as a check against the car's slightly wonky mpg calculation and it's a difference of 5ish mpg more for highway like the EPA estimates suggest.

(mi/wh drops. wh/mi increases.)

Rocks slow down due to air resistance :-)

Drops like a meteor?

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