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Comment by jnsaff2 | Hacker News Reader

Also, people in the EV conversion world tended to think that weight was the killer of efficiency. They later found out by experience that weight was much less important than wind resistance with the conclusion that speed is mostly killing your efficiency (along with shape of the car).

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I’ve noticed this recently in my driving. As an experiment, I lowered my highway speed from 75mph to 65mph for a couple weeks, and my gas mileage went from about 26mpg to 34mpg. Car is epa rated for 33.

An enormous improvement in efficiency - that experiment has totally changed my driving habits. Knowing that, I can’t believe that I’m basically the slowest car on the road now (going 65, which is the speed limit!). Crazy to think about how much fuel we’re wasting by driving fast

You can get similar fuel savings by following a big vehicle like a truck.

Even at a safe following distance, the already moving air behind a truck lowers your effective speed by 20mph or so, saving 25-50% of the fuel bill. Trucks that have very little ground clearance (eg a big mudguard across the back) work best.

In the extreme case, in an EV, you can end a journey with more battery than you started with if you follow a truck closely (rather too close for safety, and in perfect conditions)

How would you gain energy in that situation? Surely the EV is still expending some energy? The only way that would work is if you were going down hill.

The effect of drafting though is incredible. If you're ever able to get behind a big truck on a bike, it basically becomes effortless to get up to 30mph and stay there. It's quite a thrill (just make sure you have an escape route)

I seriously doubt those numbers (-50% fuel, energy harvesting by driving behind a truck) AND you are giving incentive to try dangerous driving.

Please either back those numbers up with evidence or remove them for the sake of road safety.

On a flat (private) road, big container-hauling truck, 65mph, stick shift car following 20 feet behind... Put the car in neutral and you can follow for miles. You need to tap the brakes occasionally to stop yourself getting too close to the truck. That tells me no engine power is being used.

Obviously it's unsafe to do on the public road for a bunch of reasons, but it really shows how good automated vehicle convoys could be for the environment if they can get close enough.

Here is a video of the effects of the same airflow: https://youtu.be/xkbe2NP30Fs

Here's mythbusters doing it: https://www.youtube.com/watch?v=lttgT1XZVvE

They estimate the savings at insane 20 feet at 20 something. Far from your claimed 100%.

Could it be, that your road is not as flat as you think? That would also explain the necessary braking to not run into the truck, wouldn't it?

But I couldn't agree more: let the machines do that perfectly and safe. Let's reap those sweet 25-50% increase in fuel efficiency.

The truck they're following there doesn't go near the ground... I suspect you really need to not have any route under the truck for the airflow for it to really work well.

Make sure to not hog the fast lane :)

Haha of course I stay out of the way! :)

As an emergency response to the 1973 oil crisis, on November 26, 1973, President Richard Nixon proposed a national 50 mph (80 km/h) speed limit for passenger vehicles and a 55 mph (90 km/h) speed limit for trucks and buses

https://en.m.wikipedia.org/wiki/National_Maximum_Speed_Law

My driving habits are changing when I drive my wife's hybrid. There's a gauge indicating the efficiency and I always try to "beat" my best: I can do as low as 2.5 l / 100 (about 94 mpg).

My gazzguzzler OTOH is not planet-friendly: about 20mpg cruising on the autobahn at 105mph / 170km/h. I plan to dump it one of these days for a (luxury) BEV.

> Crazy to think about how much fuel we’re wasting by driving fast

I agree. I can't help it yet but I'm working on it.

That's not very surprising:

kinetic energy = 1/2 * mass * velocity^2

Regenerative braking offsets a lot of the extra weight.

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