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Comment by mrfusion | Hacker News Reader

offshot topic, from reading the stats at the end of the article: How are cars with higher city than highway milage not violating conservation of energy?

I've heard the explanation that regenerative braking in city driving recharges the battery, but surely it's still better to not brake at all, right? You must incur some loss recharging the battery from kinetic energy?

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Lower speeds mean lower air resistance. City fuel economy should be higher than highway if braking energy is recovered at a high enough efficiency.

Good point. So really the best milage would be going a constant but lower speed? Like 50MPH?

Seems like it: http://en.wikipedia.org/wiki/Energy-efficient_driving#Mainta...

Yep. Don't confuse "highway" for "best". My Prius v is rated at 44 city and 40 highway. If I drive it for extended periods on back roads (i.e. mostly around 55MPH) I can easily get 60+MPG. Optimal speed for any given car will vary depending on the engine, the gearing, and other such things, but typical highway speeds are usually beyond it. This is especially true with hybrids, which can reduce or eliminate a lot of the efficiency losses that normal cars suffer from at lower speeds.

That's remarkable. My Prius V (2013) gets about 49 mpg when I drive 55 MPH on smooth, level roads. A couple of times I've hit 50 mpg. Also, I frequent the Prius Chat forum and I've never heard of anyone getting 60+ mpg. Have you modified your vehicle? Is it a plug-in?

Weird. It's as stock as stock can be. I get 48-50MPG in my normal mix of city/interstate driving. That's on the clock, so de-rate by 5% or so for real MPG as calculated by miles on the odometer divided by gas purchased. Best I saw was 65MPG on the clock over a ~2 hour drive on various country roads coming back to my house, and not flat either (but a pretty minor net elevation change).

http://cdn.teslarati.com/wp-content/uploads/2014/05/Tesla-Ra... -- this is the diagram for the non-P non-D version. Peaks around 25mph.

Most gasoline cars are designed to get best overall efficiency cruising at 55 MPH. That's kind of the crossover point where wind resistance is not crazy high and the engine is still getting good efficiency. So you're burning more gas per minute than you would be in a city, but you're covering a lot more ground per minute as well, so your gas-per-mile is very low.

Drag increases as the square of velocity. https://en.wikipedia.org/wiki/Drag_equation

It is better not to brake at all, and if you drive around in circles at low speeds in a light, tiny, aerodynamic car you can literally get thousands of miles per gallon[1]. Even an efficient car like a Tesla is not close to a theoretical limit, so conservation of energy is not an issue.

[1] http://en.wikipedia.org/wiki/Eco-marathon

In general, ICEs are very efficient at a constant speed (highway driving) but much less efficient when accelerating and breaking is a net loss of useful energy.

EVs have a different sort of performance profile. They're far more efficient at accelerating, they partially regenerate their energy when braking, but they use a lot of energy when cruising at highway speeds.

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