How much fuel would a national 55mph limit save a year?
October 13, 2008 12:49 PM   Subscribe

If we lowered the speed limit nationally to 55mph, how much fuel would we save?

I've often heard about how much more efficient a 55 mph national speed limit would be. But would much fuel would it save nationally over a year? Percentage or number of gallons would both be useful.
posted by mikeyk to Travel & Transportation (31 answers total) 3 users marked this as a favorite
 
None since people don't bother driving the speed limit anyway. Revenue from speeding tickets would however increase...
posted by foodgeek at 1:06 PM on October 13, 2008 [2 favorites]


foodgeek - I would assume that people on average drive some percentage over the limit. So, people might drive 80 in a 65mph zone but I bet they wouldn't drive 80 in a 55.

But Sammy Hagar couldn't even get it up to 55.
posted by ian1977 at 1:12 PM on October 13, 2008


Drive55.org says that it would save 10% on fuel by imposing a national 55mph speed limit, but foodgeek is right; we'd be better served actually physically limiting cars to 55mph, because signs are so often ignored.
posted by sjuhawk31 at 1:13 PM on October 13, 2008


That article can be confusing, but conventional wisdom is that you lose 1% of your mpg for every mph over 55.
posted by sjuhawk31 at 1:15 PM on October 13, 2008


This question is somewhat like asking "How much would re-enacting Prohibition save in health care costs?" The US already tried this and it didn't work very well.

Wikipedia also notes that the Heritage Foundation found studies in 1986 that suggested the fuel savings were between 0.5 and 1 percent, but I don't exactly trust the Heritage Foundation to be objective.
posted by burnmp3s at 1:16 PM on October 13, 2008


Drive55.org says that it would save 10% on fuel

you mean on emmisions

This editorial
claims that the last time we saved 167,000 barrels of oil a day., and that now it would be even more.

However, this is the land of the free and speed limits, especially super slow speed limits like 55 mph, just seem so UnAmerican.
posted by caddis at 1:19 PM on October 13, 2008


The idea that 55mph is somehow the mythical most fuel efficient speed is also flawed. Some cars may use more fuel to run that slow as they are designed to cruise efficiently at 65 or 70 mph through gearing choices.

So the question is kind of random and moot - if all cars were geared to be more efficient at 55 than any other speed, then limiting them (through legislation and associated strong arm tactics) to that speed would save money, but until all vehicles run at their most efficient at that speed, quantifying it will be extremely difficult.

So anyone that claims an answer is either ignoring some variables, or only giving a half answer. Like an awful lot of wild claims, the speed limit is only a small proportion of the issue. Finding an objective source in this one is exceedingly hard, to (as has been noted).
posted by Brockles at 1:21 PM on October 13, 2008 [3 favorites]


I would expect that most European cars are tuned to have optimal fuel efficiency well above 55 MPH. The speedometer in my Jetta is straight up at around 110 kph which I take to be a vague approximation of expected highway cruising speed.
posted by GuyZero at 1:26 PM on October 13, 2008


Some cars may use more fuel to run that slow as they are designed to cruise efficiently at 65 or 70 mph through gearing choices.


Really? Name one car that is more efficient going 70 mph or even 65 mph than 55 mph. It isn't about the gearing or any other mumbo jumbo. It is about the wind resistance and that increases as the square of the speed. 65 squared is about 40% more than 55 squared. The drag at these speeds dwarfs other power drains. I do agree though that there is nothing magical about the 55 number.
posted by caddis at 1:34 PM on October 13, 2008 [1 favorite]


Caddis, of course gearing affects it: it is about peak torque and peak efficiency of the engine versus aerodynamic drag. The engine may run at an efficiency greater than the extra drag required to run at that extra speed.

The drag at these speeds dwarfs other power drains.

Really? 40% of something is instantly dwarfing, without considering actual drag numbers versus torque? Interesting.
posted by Brockles at 1:48 PM on October 13, 2008


Some cars may use more fuel to run that slow as they are designed to cruise efficiently at 65 or 70 mph through gearing choices.

None that I have ever driven. I have, across the board, noticed serious declines in MPG when driving more than 60 mph in a variety of rental models. Honestly, I am inclined to believe the ~1 mpg per mph over 65 estimage, or something close to that.

(I seem to recall the Car Talk guys, many years ago, saying the ideal fuel efficiency speed for many cars was about 35 mph, and that should be the national speed limit. Then they laughed and laughed and laughed.)

Related Car Talk forum thread http://action.publicbroadcasting.net/cartalk/posts/list/21/1318009.page
Related AskMe http://ask.metafilter.com/96321/Does-the-optimal-speed-for-fuel-efficiency-vary-by-car-model
Related HowStuffWorks http://auto.howstuffworks.com/question477.htm
posted by aught at 1:49 PM on October 13, 2008


None. Many cars are more fuel efficient at higher speeds.
posted by wackybrit at 1:53 PM on October 13, 2008


I can't name a specific car, caddis, but I've seen many charts of cars that make the same mileage whether going 55 or 70.

What it comes down to, below the "magic" speed for an engine/gear combination, the brake-specific fuel consumption may fall off faster than the drag force does (I'm fudging units here). Likewise, if the efficiency goes up with the drag within a certain range, you might as well drive at the rate at the top of the range - your mpg won't be affected.

My impala has highway gears. 2.56:1 final ratio. At 60, when I punch it, it bogs a little. At 70, when I punch it, it takes off. Strangely enough, the car gets the same MPG at 60 or 70. (the mileage at either speed, incidentally, is crappy.)
posted by notsnot at 1:54 PM on October 13, 2008 [1 favorite]


One should not limit cars to the same speed as the limit.

In a large proportions of accidents which are avoided, someone gets out of trouble by accelerating away from it.
posted by genghis at 2:01 PM on October 13, 2008 [2 favorites]


foodgeek - I would assume that people on average drive some percentage over the limit.

No, people drive the speed the road is designed for; they intuitively know what a safe speed is based on the vehicle they are driving and the road's characteristics. A few people drive too fast; a few people make an effort to drive near the limit. But most people drive the design speed of a road, and it's very difficult to change that.
posted by Dasein at 2:01 PM on October 13, 2008


people drive the speed the road is designed for; they intuitively know what a safe speed is based on the vehicle they are driving and the road's characteristics.

You give the average driver far, far too much credit. Especially in terms of understanding their vehicles. I'd say the vast majority have zero idea how their car will behave at the limit or in a sudden avoidance manoeuvre. Nor do anything like a trace percentage have any concept of the 'design speed' of a road.

I think people drive the speed limit + a nominal amount they think they can get away with/won't lose their license for.
posted by Brockles at 2:14 PM on October 13, 2008


Caddis, of course gearing affects it: it is about peak torque and peak efficiency of the engine versus aerodynamic drag. The engine may run at an efficiency greater than the extra drag required to run at that extra speed.

From anecdotal experience, I'd say "not enough". I would be shocked if there was any mass-production car so incredibly poorly designed that its gearing was so bad at 55mph that it got better mileage at 70mph.

Hop on a bicycle. Get up to 30mph on a flat bit of road. It's a bit of work, but you can do it.

Now, find a long steep downhill straight, and don't pedal. Now you can get up to 30mph without doing any work at all! Now pedal to increase your speed, and you start to appreciate the scale and solidness of the problem. Even when you get up to a mere 40mph - just a puny 10mph faster, you'll be working and you'll feel how much a barrier it is to try to force such large volumes of air to move around so suddenly and so quickly. At just 45mph, like trying to wade quickly through water, your muscles will be straining against the force pushing into your chest, and every additional mph will come at such increasing cost that if anything, 55mph seems hopelessly inefficient.

Any regular car that gets better mileage at 70 than 55 would be a joke of engineering incompetence to have such massive inefficiency and vast unnecessary energy losses in the 55mph bracket. Why not just put a leak in the fuel tank while you're at it? The extra energy required to go 70 is significant, and to be wasting that much extra energy for no gain at 55 is ridiculous. But with Detroit's reputation, maybe such a car exists?
posted by -harlequin- at 2:34 PM on October 13, 2008


I will concede that the 1997 Toyota Celica gets better fuel economy at 65 mph than at 55 mph (see chapter, four page 26)
posted by caddis at 2:44 PM on October 13, 2008


You are, of course, aware that cars are not all built in or by Detroit and can be made to be optimised for a market different to the US.

Crazy, I know. But feel free to check out some of the comments here that was linked to above. If a car is geared for a very high speed (so any decent performance car) it will be running quite low in the rev range at 55mph as it is pretty damn slow. It may well be better for the engine efficency to be in a more sensible rpm range at slightly higher speeds.

I'm not saying that all cars somehow get more efficient as they go faster, it's just that there will be speeds below their peak efficiency just as much as above. There is nothing to suggest that 55mph is that peak efficiency speed. Also, there may be no benefit from dropping to 55mph - the drop in aero resistance may match the drop in torque, so why limit that car from going faster?

Any regular car that gets better mileage at 70 than 55 would be a joke of engineering incompetence to have such massive inefficiency and vast unnecessary energy losses in the 55mph bracket.

What a ridiculous thing to say. No-one is suggesting that it will be doing 10 mpg at 55 and 70mpg at 70 mph.
posted by Brockles at 2:51 PM on October 13, 2008 [1 favorite]


people drive the speed the road is designed for; they intuitively know what a safe speed is based on the vehicle they are driving and the road's characteristics.

You give the average driver far, far too much credit. Especially in terms of understanding their vehicles. [...] Nor do anything like a trace percentage have any concept of the 'design speed' of a road.


Near me there are some weird roads. There's a dual carriageway which has all the characteristics of a road with a 50mph or 60mph limit, but it has a 30mph limit. I don't know why.

I drive slowly on that road because the police regularly set up speed traps. Police regularly set up speed traps because a lot of people speed on that road.

On the other hand there are narrow, winding country lanes one can theoretically do 70mph - except for the fact you would die at the first corner.

I think this is what Daesin means; people only exceed the speed limit when and by the amount they consider it safe. Admittedly, not everyone has perfect judgement in this, but most people are fairly reasonable.
posted by Mike1024 at 4:10 PM on October 13, 2008


we'd be better served actually physically limiting cars to 55mph

We absolutely would not—this would kill a large number of people.
posted by oaf at 4:10 PM on October 13, 2008


I think this is what Daesin means; people only exceed the speed limit when and by the amount they consider it safe.

I stand by my assertion that the vast majority of people have a completely flawed perception of what is safe, however. The issue is that they don't intuitively know what is a safe speed for a road. They just go at a pace tailored by the chances of getting caught and are otherwise tempered by staying below a speed that scares them. This has no bearing at all on whether it is safe or not.

The ignorance of the common motorist is really quite stunning at times. Tailgaiting is a perfect example. Either people need to treble (or more) the distances between them and the car in front, or halve their speed if they wish to drive so close. That they do neither proves that they have no concept of how safe it is to continue driving as they are.
posted by Brockles at 4:30 PM on October 13, 2008


Caddis, of course gearing affects it: it is about peak torque and peak efficiency of the engine versus aerodynamic drag. The engine may run at an efficiency greater than the extra drag required to run at that extra speed.

Notably, the C5 Corvette possessed an extremely tall sixth gear, which was rumoured to be purposed for optimal fuel economy (32MPG!) at 65 mph.
posted by Kwantsar at 5:44 PM on October 13, 2008


people drive the speed they guess the road is designed for; they have a self-belief that they intuitively know what a safe speed is based on the vehicle they are driving and the road's characteristics. More than A few people drive too fast; quite a few more people need to make an effort to drive near or under the limit. But most people drive what their uninformed guess is about the design speed of a road, and it's very difficult to change that without a decent commitment to effective communications.

FTFY.

Just a little bit over? You bloody idiot.

"Hypermilers" are the people to go to to get a decent answer to this question. Only two of this list of 108 tips talk about maximum speed, they're all about smooth driving and conserving momentum.

It's self-evident that in general there will be a saving in fuel if you drive more slowly (at the higher speeds where wind resistance is a major factor). Getting some facts about just how much would be saved appears a bit difficult because of the assumptions and generalisations that would have to be done across the entire car fleet. It doesn't appear that anyone has done this work - it would eb a major piece of work you'd need a government to sponsor.
posted by wilful at 5:59 PM on October 13, 2008


Really? Name one car that is more efficient going 70 mph or even 65 mph than 55 mph. It isn't about the gearing or any other mumbo jumbo. It is about the wind resistance and that increases as the square of the speed. 65 squared is about 40% more than 55 squared. The drag at these speeds dwarfs other power drains. I do agree though that there is nothing magical about the 55 number.

1997 Pontiac Grand Prix GTP.

THe reason a car might get better fuel mileage at speeds higher than 55 is that gasoline engines are horribly non-linear in their efficiency. If it takes 30 HP to go 55 and it takes 45 HP to go 70, and the engine consumes less fuel making 45 HP, then it will be more efficient.

Physics is linear, engines aren't.
posted by gjc at 6:01 PM on October 13, 2008


For the record, my car--which is German--exceeds its EPA estimated highway mileage to the tune of 2-3 MPG on trips as short as 250 miles when I set the cruise control to 70-75 MPH. That includes full-throttle, tire-spinning burnouts coming out of rest stops and toll booths, naturally. And 20-30 miles of slower stop-start driving.
posted by uncleozzy at 6:17 PM on October 13, 2008


Name one car that is more efficient going 70 mph or even 65 mph than 55 mph

Thanks to a very narrow, turbo-enhanced powerband, an intolerance for RPMs lower than about 1500, and a pretty large gap between 4th and 5th gear, my 2002 WRX wagon fits this description. To drive at 55 for any length of time smoothly would virtually require 4th gear. 65-70 is ~2000 RPM in 5th, a good cruising speed with very little turbo boost, and yields the best mileage in this particular car.

This is probably by design. Buyers of this particular vehicle aren't likely to spend much time below the speed limit. Why pretend otherwise.
posted by toxic at 6:19 PM on October 13, 2008


Buyers of this particular vehicle aren't likely to spend much time below the speed limit.

Maybe, just maybe, it's because for most of the market that car is aimed at 65-70mph IS below the speed limit. God, a massive amount of the States has a 65 or 70mph speed limit on the freeways, and the UK and most of Europe is comparable. The US 55mph speed limit isn't a major consideration for international car manufacturers.

Until all cars are designed for a 55mph speed limit, or one where the whole world has that same limit (to get back to the original point) it is unlikely that this will be the point of optimum efficiency for a cars economy.
posted by Brockles at 6:37 PM on October 13, 2008


You are leaving out the part where people waste gas driving badly in stop and go traffic at low speeds.

Jumping off at green lights, slamming to a stop a red lights, cornering stupidly.

Not sure where the average miles for the average driver are, but I do know that commuter traffic and city driving is not at 55 and people manage to waste gas.

For the record, my car which is old and American, exceeds it's original EPA by 5 mpg in the city with the AC on. When it was younger, it was way better. But that's because it's a manual and the EPA method back then was to have the test driver switch gears at the same time an auto would switch - which has nothing to do with how people drive. Last I heard, they where phasing out testing on manual cars. I no longer have any contacts there so I'm not sure what's up now.
posted by Lesser Shrew at 8:30 PM on October 13, 2008


I'd like to see the EPA testing be a little more thorough, and advertise the methodology better. "If you drive like this, you'll get this MPG. But if you drive like this, you'd get this other, worse MPG." Put it on the Monroney sticker.
posted by gjc at 7:57 AM on October 14, 2008


I just flew in from Florida and boy are my arms tired...

This argument is flawed because in some places, like western florida, the land is flat as a flitter, and in some places like here, it's almost vertical. The 2wd Ram 1500 I drove down there got better mileage than my 4 cylinder stratus does up here...because it's not hauling ass up hills.

My mom's got a late 90's grand am, it does better at 67 than at 60. My gf has an early 2000s grand prix, it does better at ~70 than at 55 or 60 because it has trouble finding a gear to sit in. My mom used to have a 6.9 Liter GTO and, not surprizingly, it got better gas mileage at 72 than at 55 (which was the speed limit at that time.) Around me, a LOT of 8cyl's get better highway mileage than 4 bangers, because the hills rape the low-torque motors at high speed.

I believe, and call me crazy, that this is the first step to badass fuel mileage:
-variable cruise control (5-10mph band where you set min/max and car decides for you based on engine load.)
-GPS aided cruise control (car knows big hill coming, so rather than just coasting down the last one it speeds up a little bit to gain momentum for the next hill)
-cloud GPS/turn-by-turn/cruise control (your car knows what the cars in the next 1-5 miles are doing and adjusts accordingly)
-drafting (I got 36mpg out of my stratus a couple weeks ago, I stayed behind a semi for a couple hundred miles!)
-intelligent traffic control systems (as discussed in Wired, systems that dynamically route traffic from high-priority to low-priority areas based on immedaite congestion and route availability.)
-interlinking that system with your car gps/turn by turn/cruise control

So there.
posted by TomMelee at 7:58 PM on October 15, 2008


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