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Comment by isalmon | Hacker News Reader

Very good concept overall, I like the idea A LOT. One thing that surprised me: >> Speeding doesn't save as much time as you might think, but driving 10 MPH over the speed limit reduces fuel efficiency by 12-15%.

How's my fuel efficiency related to the speed limit set by local authorities?

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Hey isalmon, fuel efficiency is not related to the speed limit. We just use that as a frame of reference. The actual numbers are that fuel efficiency drops by 12-15% for every 10 mph above 50 mph. Here's an article which talks about that - http://www.greencarcongress.com/2013/01/thomas-20130117.html

Well, I guess you should clarify it then on the website? Does not really help your credibility ;)

That data includes cars going back to 1984. From what I have heard (and have no citation for this), newer cars remain efficient at higher speeds than 50 mph. Do you know if that's true?

The biggest change is air resistance.

Some day when you are on a freeway without many other cars around, shift into neutral. Depending on your car model, you will lose 1-2 mph every second until you get to around 50 mph, when you will lose speed much more slowly. On a flat road it might take you a whole minute to drop the next 10 mph.

(Make sure you are in the right hand land before doing stuff like this. Also be aware that driving too slow can be as dangerous as driving too fast.)

I wonder if some wing tech were possible to be used on vehicle body as an aftermarket option to increase laminar flow (or delay separation, I'm not sure, not an AE) to reduce air resistance.

I've seen hypermilers put aeroshells (don't know the proper name) on the backs of their cars to reduce air friction. It's pretty simple to do, but it looks very trashy.

They mean common US freeway speeds of course, shortened for copy punch/length.

The highest gear's most efficient peak of most "consumer" cars maximizes efficiency around 65mph. The NHTSA considered efficiency along with safety when making their recommendation so many years ago.

65mph is slightly too high for most models.

http://en.wikipedia.org/wiki/File:Fuel_economy_vs_speed_1997...

I think it's probably saying that every 10MPH increase has a corresponding fuel efficiency hit

Obviously it varies by car but the the theory is that most engines are less fuel efficient at higher speeds. So driving the speed limit is more fuel efficient than driving over the speed limit.

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