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Drive 55?
Back in the 70s, the oft-maligned 55 mph speed limit was the government's response to the fuel crisis. Last night the topic of reinstituting 'Drive 55' arose during some debate chat. I puzzled JaneEyrez by asserting that 45 mph was when air resistance becomes the major factor in fuel efficiency, but this Answers.com article claims an even lower speed.
The power to overcome air resistance increases roughly with the cube of the speed. Thus, above about 30 mph (48 km/h), wind resistance becomes a dominant limiting factor. By driving at 45 rather than 65 mph (72 rather than 105 km/h), the power to overcome wind resistance is about one-third, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) increases the power requirement by 2.6 times, and drastically decreases fuel economy. In practice, rather than doubling or halving the fuel economy, the difference is actually closer to 40-50%, since rolling resistance, which is broadly proportional to speed, is also a factor.
Autoblog, however, notes that wind resistance varies quite a bit with different vehicles, so an Insight or Prius would be more efficient at overcoming air resistance than a Suburban or Eighteen wheeler truck at the same speeds. (Of course, a train or bike would be even more efficient, but that's another discussion.)
Wind resistance ... is usually expressed as a number called the "drag coefficient" (Cd). While having a low drag coefficient is important, the size of the vehicle (expressed as "frontal area") is also important. In general, as drag coefficient and frontal areas decrease, a vehicle becomes easier to push through the wind. Automakers are well aware of this, so they go to great lengths to ensure a low Cd on vehicles designed for high efficiency. Some approximate Cd values for well-known vehicles.
.25 Honda Insight
.26 Toyota Prius
.27 Nissan GT-R
.34 GMC Acadia
.36 Bugatti Veyron (spoiler retracted)
.36 Chevrolet Suburban
.48 Classic VW Beetle
.65 Tractor Trailer (with fairings)
.75 Formula 1 Car (downforce = high drag)
So in my opinion, 55 becomes a rather arbitrary number particularly given the changes in vehicle design and efficiency since since the 1970s. Instead of clumsy attempts to legislate a speed limit, I'd recommend that accurate fuel use gauges be installed in cars. If people can see just how fast they're draining their fuel tanks at 75 vs 55, the smart ones will be likely to temper their speed to save money.
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Comments (15)
I seem to recall some onboard computers providing real-time MPG estimates.
Of course, reading dashboard displays obsessively while driving is a very bad idea...
October 8, 2008 10:49 AM | Reply | Permalink
They do have them in some cars now - mine shows me how many gallons I've used ever (not a fun number to look at, by the way), the average MPG, as well as a meter that measures when I'm getting better vs. worse mileage.
October 8, 2008 10:54 AM | Reply | Permalink
An on dash indicator for tire pressure will likely do more to conserve fuel. People are just to lazy to check their tire pressure on a weekly basis as per most car manuals.
October 8, 2008 11:14 AM | Reply | Permalink
Hey, it's me, JaneEyrez. I know you're doing your best to simplify this, but I still just want you to say something clear, like: Driving 55 (or any number you would chose) will save substantial amounts of gas, plus the additional benefit of saving lives.
Somebody last night suggested that it would lead to greater traffic congestion, but I don't understand that. Traffic flow is traffic flow, regardless of the speed, and the truth is that when traffic does become significantly congested, as in when you get into population bases, speeds become greatly reduced, regardless of the posted limit, because people, quite frankly, don't want to smack into the car in front of them.
Hey, but thanks! Imagine my shock that I'm being referenced in a blog that I didn't personally write. A thrill, indeed.
October 8, 2008 11:32 AM | Reply | Permalink
There have been studies which show that moderate (not slow) speeds actually decrease congestion on highways because it keeps everything moving more evenly (rather than start-and-stop spring-like motions).
October 8, 2008 12:35 PM | Reply | Permalink
All I can say is that driving slower is more energy efficient than driving faster, even in a Tesla.
Most congestion could be solved if people would simply reserve the left lane for passing. That won't happen, of course.
October 8, 2008 1:12 PM | Reply | Permalink
While air resistance can play a factor, the 55 mph number is based on engine efficiency. Engines generally become less efficient around 60 mph. Often your driver's manual will tell you this. Drive over 70 mph and things quickly deteriorate in terms of miles per gallon.
Rec'd for continuing to remind us about energy being the single most crucial issue affecting us -- until it becomes water.
October 8, 2008 12:33 PM | Reply | Permalink
I'll bet that's all changed, too. Since the 1970s, there's a wider variety of engine displacements from tiny three cylinders to absurd twelve cylinders to no cylinders (EVs). (That thing got a Hemi?) Turbo charging has become much more common and automatic transmissions are more efficient than most drivers with manual.
October 8, 2008 1:02 PM | Reply | Permalink
At 58 steady mph my car gets 41 mpg. It is EPA highway rated at 32. I use the usual economy driving techniques and do not impede traffic. The economy indicator has trained me well.
Along with the correct tire pressure, a fresh air filter element can give an immediate jump in mileage (and performance). Most of the air filters are cheap and easy to change. Take a look under the hood and consult your owners manual.
We have not made much progress. The Ford Model T got 25 mpg.
Regards to all.
October 8, 2008 1:14 PM | Reply | Permalink
Some links:
http://autospeed.com/cms/A_110216/article.html
http://ask.metafilter.com/96321/Does-the-optimal-speed-for-fuel-efficiency-vary-by-car-model
http://community.discovery.com/eve/forums/a/tpc/f/7501919888/m/9901907189/p/2
October 8, 2008 1:33 PM | Reply | Permalink
Hard to make the speed limit work, although it is not in dispute that it makes sense in every possible way (fuel, safety, traffic).
Two factors fight it. First is a tragedy of the commons, where only some follow the law, there not being enough police to enforce reliably. Safety, fuel costs, and traffic flow are compromised by even a minority.
Second, there are not enough police because the roads and cars feel so easy at high speeds that everyone drives in at least the 70 mph range on modern highways, disregarding 55 mph limits near cities.
October 9, 2008 12:10 AM | Reply | Permalink
Safety is not the issue. Modern cars and highways are much safer than they were even 20 years ago. Driving slower doesn't not necessarily mean driving safer. I prefer to drive fast and I will continue to drive fast. That's why I use a radar detector. Studies show that drivers who use radar detectors are safer drivers.
Fuel efficiency is more complicated than just driving slowly. There is an optimal speed for fuel efficiency. Driving very fast becomes inefficient, just as driving very slowly does as well. A fast car suffers from drag. A slow car suffers from increased time to cover the same distance. There is an optimal speed for efficiency, but it varies greatly for different cars and even different atmospheric conditions.
Driving style affects efficiency as well. I drive a manual transmission which is more fuel efficient. Hypermiler techniques like pulse/coasting and "brakeless" driving can dramatically increase efficiency.
And in the immortal words of Sammy Hagar -- "I can't drive 55!" Seriously, I will get a radar jammer if I have too, and I will also ditch my Ipass if they start timing me through the gates.
October 9, 2008 2:11 AM | Reply | Permalink
I was driving down two-lane Rt 40 on Monday. In my rearview I saw this guy pass a school bus on the double line, he tried to pass me, but someone was oncoming, then he passed going about eighty. It was two guys in an old white Corsica with a racing logo. He disappeared ahead, but I pulled up behind him at the first light on Frederick's main shopping drag. I guess he had to race on the open road, because he consistently picked the slowest of three lanes in traffic.
October 9, 2008 8:28 AM | Reply | Permalink
Of all the things, driving 55 is just a non-starter. Sorry. It's hell on earth.
I'd rather just do the whole return-to-the-stone-age thing now and skip the 55mph stage.
October 9, 2008 9:38 AM | Reply | Permalink
I think a lot of people would agree with you.
October 9, 2008 12:45 PM | Reply | Permalink
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