Why are cars built to drive twice as fast as the speed limit?

[Kaleva]

Would there be a significant savings in fuel and lives if every vehicle (other then law enforcement and emergency vehicles) had digital controls which limited top speed to 55 or 60 mph?

[elocs]

1. Yeah, I see that happening real soon. n/t

[hobbit709]

2. Sometimes you need the acceleration capability.

[Kaleva]

4. acceleration and speed are two different things.

I haven't had a justifiable reason to drive over 100 mph. Over 30 years ago, the judge didn't accept my excuse I was fleeing from the police and I ended up with jail time.

[hobbit709]

17. Have you ever driven a vehicle with a governor on it?

It gives the vehicle lousy acceleration.

[Kaleva]

25. I'm thinking of something different then a governor.

The vehicle still has the acceleration as it normally would but once it reaches a certain speed, it behaves as if one take their foot off the gas pedal even though that person still may have the pedal to the metal.

[madokie]

38. Thats what a governor on an engine does

if you put the petal to the metal its balls to the walls up to the governors set rpm, Acceleration is all there top speed is only limited, not the power. Obviously if you're traveling at the say 60 mph and you floor it nothing happens but drop down a few mph and floor it and you return to the governors set speed as fast as the engine can do it. in other words WOT, wide open throttle.

If you want to limit the top speed of an vehicle's engine a governor is the way they do it. It can be electronic or it can be mechanical, both does the same thing, limit the top speed.

[GliderGuider]

42. Especially an electronic speed limiter

The one on my 2000 BMW 540i kicked in at 250 kph (155 mph), at which point it cut off the fuel injectors. Up until that point it had no performance impact whatsoever. I only had that happen once...

[madokie]

37. You misunderstand what a governor is for, methinks

a governor on an engine will only limit the rpms of said motor not acceleration, I repeat a governor does not limit acceleration, it only limits top rpm's. A diesel is the only engine that the auto manufacturers put a governor on so I think the lousy acc you're refering to is due to that not the governor. :hi:

[ngant17]

8. that's definitely true

if you are towing a load or have a trailer, and if your load starts swerving unexpectedly, you should immediately increase your speed to reduce the instability and to get the load under control. Slowing down will only make it worse.

You would always want some kind of reserve power capability at the top speed.

[Kaleva]

9. The U-Haul website says to slow down if trailer starts to swerve excessively.

It also says that excessive swerving is most likely caused by an improperly loaded trailer.

[One_Life_To_Give]

21. Guess you never trailered much

If you ever have a trailer start to Jackknife on you and hit the tow vehical brakes. Expect to have the trailer introduce itself to you right quickly as it joins you in the passenger compartment.

[Kaleva]

23. Where did I say "hit the tow vehical brakes"?

Also, the poster I replied to and I were talking about the trailer "swerving". I've towed many a hay trailers (which didn't have brakes) in my younger days and a certain amount of weaving was normal. However, when the trailer began to whip (which is different then weaving) I was going too fast and slowed down by taking my foot off the gas. If you've ever seen people hauling hay trailers, you'll have noted they weren't driving anywhere near 55 mph and that's because trying to do so would be a good way of flipping the trailer.

[ngant17]

29. maybe UHaul wants the customer to pay to replenish their inventory

by misleading the customer into dangerous driving practices in which the corporate office will not be accountable for. After all, the ticket's on you.

Physics isn't very democratic, either!

[ngant17]

30. all that is corporate legalese

to get you to pay for any damages. If you are towing a load at high speeds, you will definitely increase the instability of a fishtail situation if you hit the brakes. You need to accelerate until the swerving begins to correct itself. I originally got this advice years ago from my brother-in-law, who frequently tows a large, expensive boat for fishing on weekends.

[ngant17]

31. granted that there is a lot of advice both ways

but I would tend to believe this:

http://www.mpi.mb.ca/PDFs/DVL_PDFs/DVLTrailerSafety.pdf

...the car and trailer swerve. Braking will not help. Grip the steering wheel firmly and accelerate slightly to bring the unit under control."

[Kaleva]

32. It says to accelerate slightly

It also says to avoid high speeds and if traffic builds up behind you, to pull over and let it pass. An interesting and informative site and i thank you for posting a link to it!

I'm only familiar with towing farm equipment which we had to hang slow moving vehicle signs on the back of as they never were designed to be towed at high speeds.

[Kaleva]

3. Some info.

"While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.

You can assume that each 5 mph you drive over 60 mph is like paying an additional $0.24 per gallon for gas.

Observing the speed limit is also safer."

http://www.fueleconomy.gov/feg/driveHabits.shtml

"So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed."

http://auto.howstuffworks.com/fuel-efficiency/fuel-economy/question477.htm

[oldhippie]

15. My Dodge/Cummins 3/4 ton diesel truck gets max fuel efficiency ......

.... (or Brake Specific Fuel Consumption (BSFC)) at 2000 RPM. With my top gearing and tire diameter that's 70 mph. That is also the normal speed limit on most roads around here, so it works out just right for me. I can generally get 19-20 mpg at 70 mph out on the hiway.

[AtheistCrusader]

45. The engine perhaps.

But wind resistance increases with the cube of your velocity. You WILL burn less fuel if you drive 50mph.

Not that I advocate reducing the speed limits, do whatever.

[TxRider]

5. Most definitely, likely 20-30% or more savings or more

[madokie]

6. Back in the nixonian age

we went to a 55 mph speed limit and brother let me tell you it pissed everybody including the cops off. During that time I learned all about hypermiling even though I didn't know what it was called and still not sure how its spelled.

[doc03]

7. Because I don't like speed limits n/t

[tinrobot]

26. Some don't like limits on...

The amount of trees they can cut down... The amount of water they can pollute... The amount of fish they can take from the sea... The amount of CO2 they can pump into the atmosphere.

Doesn't mean that those limits shouldn't exist.

Speeding sure is fun, and cuts a few minutes off the commute, but it dumps a lot more much pollution and CO2 into everyone else's atmosphere.

[AlecBGreen]

10. 120 is the MINIMUM safe speed

to drive through a horde of zombies



let me tell ya, if try to drive through this crowd @ 55 you'll get bogged down with broken limbs, bodies etc. They'll pull you out and eat you.

[Kaleva]

14. The drummer for "The White Stripes" is a zombie?

[RC]

11. Yeah, a top speed of 55 or 60 will work real well when the speed limit is 75 mph.

And any place of any significance is 60 to 100 miles apart on a road with a design speed of 110 mph.
Pedal faster.

[Kaleva]

13. The Interstate has a design speed of 70 mph. What US highways have a design speed of 110?

"Design speed should generally be 70 miles per hour, with 60 miles per hour sections allowed in hilly terrain and 50 miles per hour sections allowed in urban areas or within mountainous regions."

http://www.interstate-guide.com/interstate.html

"Generally, for freeways and the Interstate System, the design speed shall be 70 mph for either column shown in Table 2-1. But in certain urban areas, the Interstate highway or freeway was designed at 60 mph, therefore the design speed shall be 60 mph in either column for these areas."

http://www.state.nj.us/transportation/eng/documents/RDM/sec2.shtm

[RC]

34. Not true. You do not design a road for a top speed of 70 mph and have a speed limit of 75 mph

You need to allow a margin of safety. Most places the interstate design speed is 110 mph and in places higher yet. Others it is lower, but it is always at least 30 mph over what ever they decide the posted limit will be.
You are talking about what they decided the speed limit will be. I am talking the safe psychical design speed limit. That is generally 110 mph on the interstates in this country.

BTY, the posted limit on the interstates in North Dakota is 75 miles per hour, except through major cities, where it is 50 or 55 mph.

[Kaleva]

35. Most states use the 85th percentile, not design speed, in determining speed limits.

[IDemo]

12. Maybe because auto companies recognize that 80-90% of drivers

Seriously believe that "speed limit" means the absolute lowest speed acceptable, regardless of road conditions, weather or traffic.

[NYC_SKP]

16. Lack of regulation of the Industry, same reason CAFE standards were dumped in the 90's

Plenty of technological innovation went into the internal combustion engine and into making cars lighter and safer.

Unfortunately, while all these improvements could have given us a fleet average of over 30 mpg easily, it all went into sexy, high horsepower, high accelerating vehicles that the industry thought they needed to sell cars.

It was technology squandered.

[wuvuj]

19. Most cars are overpowered...

...reducing the mpg they get. For instance...a 2400 lb car would be usable if it had maybe 100 hp. 2400/100 = 24.

Instead of pandering to car buyers fantasies of "freedom" and major hp....there should be some kind of tax on high hp vehicles.

When gas prices hit $4 again...there will be?

[One_Life_To_Give]

18. Could then make a fortune removing them!

That would be as popular as the old 55 limit. I imagine it would be as common to have the system reprogramed to remove the limit as it was for old catalytic converters to be punched out years back.

[htuttle]

20. It's because modern design priorities are fucked up

Since the widespread use of petroleum a hundred and a half years ago, one of the least important qualities of a product has been it's energy efficiency. While this has been changing as energy becomes more expensive, it still often takes a back seat to considerations like convenience, the way something 'feels' when you use it, and styling.

It struck me one day after I started commuting on an Indian scooter. I was riding a vehicle that weighed 250 lbs, had a 12.5 HP engine, and had a top speed of about 53 mph. I was getting 105 mpg as well. Meanwhile, the vehicle next to me weighed several tons, had a 250+ HP engine, and a top speed of over 100 mph, even though in the city, it could only go as fast as I could, and only got 15 mph.

Can you imagine even the most degenerate Roman Emperor riding around in a massive steel carriage with a train of 250 horses pulling it? I can't.

Most of us are driving WAY too much vehicle around.

ON EDIT:

I wanted to add that "acceleration" is the most energy expensive, yet least important factor in our transportation choices. It's the one of the first things we're going to have to give up post-oil age.

[The2ndWheel]

22. Cheap energy

[tinrobot]

24. Because you need all that power in an "emergency"

At least that's what all the muscle car driving gearheads tell me...

:sarcasm:

[Kaleva]

27. I was a member of the local fire dept. and by law, we couldn't drive faster then the speed limit.

We had to obey all traffic laws just as everyone else lest we got into an accident and the department ended up getting sued.

[One_Life_To_Give]

28. Why Not

There are laws regulating how fast a vehical can travel upon the public roadways. And mechanisms in place to enforce that. However a private vehical can also be driven upon private roadways which are not subject to such restrictions. Examples would be New England Dragway has had open nights for people to take ordinary vehicals and put them on the track.

Seriously your argument comes off as Since people still speed although it is illegal/unlawfull and the Police have been unable to sufficiently stem this from happening. A solution to force everyone into compliance is proposed. While were at it why don't we force everyone to use a debit card for all transactions so big brother can track the movement of all goods and eliminate Black Markets/Bartering etc. Where does the power of Big Brother end?

[Merchant Marine]

33. Digital limiters can be defeated

by little more than a $50 flash kit. Most serious tuning enthusiasts use an ECU "flash" or re-programmer to adjust engine timing for better engine output and efficiency. They have the beneficial side effect of removing all digital speed govenors- its like re-formatting your computer to get rid of a virus.

ECU flashes can also improve fuel economy- I got a flash unit for my 2002 Explorer that optimized the engine timing for unleaded 87 octane and gave me a 3mpg improvement.

On the performance side, my buddy's 2005 Rabbit went from having a 115mph governed top speed to 140 mph. He also saw a 15 hp increase.

Your proposal is a load of authoritarian crap that would stop no-one interested in going fast.

[krispos42]

36. Thrust versus drag

If a car's top speed was drag-limited to, say 70mph (maximum speed limit in the US, I think) then the car would be very under-powered. Probably not more than 70hp for a Taurus-sized car. Acceleration would be anemic, and it would slow drastically on hills and on-ramps.

And the gas savings would be minimal. If a Taurus needs 50hp to the wheels to break the wind resistance at 70mph, then putting in a smaller (but technologically equivilant) engine really won't help that much beyond it would be a few dozen pounds lighter. The engine would still need to burn enough gas to make 50hp.

If the engine of the Taurus was replaced with a rocket engine, the rocket engine would still have to make enough power (50 hp, not sure what that would be in terms of thrust) to break through the wind resistance.


There's not a whole lot of options there. You can reduce the frontal area of a car (shorter, narrower, or both) and improve the aerodynamics (sleeker) and reduce mechanical drag (low-rolling-resistance tires, better wheel bearings), and you can improve the engine (more valves per cylinder, variable-valve timing, better lubricants, higher compression ratio, better fuel injection) and transmission (more gears, lockup torque converter, better gearing ratios).

But there's a reason that big powerful modern cars like a Corvette have highway milage not that much lower than a generic family car... when you're just cruising you're only using a fraction of the engine's power potential.

[madokie]

40. If a person stays in the engines comfort zone,

IE its torque range and the closer you can keep the engine turning at that maximum torque readings rpm you'll get the better mileage. So in engineering they try to keep those two things as close together as they can, perceived speeds one will travel matched with the max torque. Take a ford 300 inline six for instance, it produces 80% of its torque at a low 1200 rpms and its max around 2200 rpms. That's why they were/are so nice to drive. You can be trucking along at say 50, maybe 55 and want to pass someone just pull out and give it some throttle, no need for a downshift or any of that, just give it some gas and go around them and on your way. I'm a big V8 kind of guy who likes to hear it talk but when it comes right down to comfort in driving I'll take the afore mentioned six any day. All because that torque range is right where it coincides with the speeds I drive so closely. My big v8 has to get up and do a little humping to do the same thing that the lowly ole six just goes on about its way of doing things. But I like 'm the rumble of the 5.4 V8

Actually I'm pretty much done with that kind of driving anymore though


splchk

[krispos42]

41. Yeah, those older pushrod, 2-valve-per-cylinder engines had very low torque peaks

When fitted with modern fuel-injection systems, they were actually pretty respectable on the highway milage. They just lacked power at higher RPMs, while more modern overhead-cam engines with 4-valves-per-cylinder make more power with a peak significantly higher in the RMP range (assuming similar displacement and technology, of course).

But once they retrofitted those old pushrod engines like the GM 3800 and the Ford 3.8L, they got perfectly good highway milage. My 1989 Olds Regency (full-size sedan) consistantly got 22mpg on the highway with cruising speeds of 80 mph, and my 1987 T-Bird got about the same thing. Considering how big the cars were, the comparatively low compression ratio, and the older fuel-injection systems (the T-Bird used a throttle-body fuel injector), that's not too bad at all. My modern Subaru Impreza sportwagon gets about 27 with a stick.

[Odin2005]

39. To satisfy people's egos.

[GliderGuider]

43. To get technical about it, it's mostly to satisfy their ids.

The id is where all our childish lusts live, and the craving for speed and status are pure lusts.

Cars like that also satisfy our egos because they are attempts to fill the psychic hole of our our inner need for personal worth by filling it with stuff.

[flvegan]

44. There are very very few people that will drive 110 or 120 mph at any given time.

At least on most well-used roads.

Personally, I have no problem with folks driving 80 mph or so, so long as both the driver and the car have the ability to handle it with ease and the brains to know when NOT to do it.

And can we be honest here? Ever been in a populated area on the interstate when it has a 65 mph limit and there's a cop driving it as well. See how the traffic bunches? That's because everyone is doing the speed limit. Imagine that 24/7.

[carterson2]

46. variable speed limits

Around St Louis they have variable speed limits.
IMHO, if they raised the speed limit +5 at night, all the truckers would take advantage of that and drive more non-rush hour.
My 2 cents on traffic improvement.

Hey Democratic Underground. If you really want to do something unique to fight authority, help us at
wiki-speedia

Its a shared database of speed limit sign locations. No BS, just open government at its finest..

jim pruett
[phone]

[happyslug]

47. The main 1970 era Automobile writer for either Pop. Mechanics or Pop. Science made the same point

Now, I am writing of an article written (and read by me) in the late 1970s NOT today but the author (I can NOT remember which magazine at that time, I had access to both Popular Science and Popular Mechanics so it could be either one) said at that time (c1978) there was no reason for anyone to drive a car with a larger engine then 2.5 liters. Yes, most cars at that time and even today are LARGER then that, but a 2.5 liter engine can get any vehicle (Except 1/2 ton and larger trucks) any power and speed they would ever need or use.

The problem has been it has been easier to sell power and speed then anything else when it comes to cars. Fuel Economy was big in the mid to late 1970s (and has again been a big issue since about 2004 when Gasoline went over $3 a gallon) but in between it was NOT (Except in Europe and other Countries with high Gasoline taxes). In simple terms the best way to reduce the maximum speed a car can go is high oil taxes (The larger the engine the more fuel you use no matter your speed, but you "need" that large engine if you want to go over 65 mph on the interstates). Yes, the Autobahn has no speed limits (Except in Urban Congestion points) but most travel in Europe is by foot, train, Public transportation or Bicycle (Through Europe, on a per capita basis, has more cars then the US today, Europeans used they cars about 1/2 as much as Americans). The reason Europe use they cars less is the high price of transportation fuel (and that is the reason Europe prefers cars with engines smaller then 2.5 liters).

An alternative method is simply to ban engines larger then 2.5 liters in vehicles under 10,000 GVW. Hummers will still have their 5.0 and larger V-8s under such a ruling (as the Hummers do today) but most people do NOT buy Hummers (The 3/4 and One Ton Light Pickup trucks outsell the Hummers in the same categories for most farmers, Construction workers, and other users of such pickups often need such trucks to haul the equipment they need and the "exceptions" used by Hummer Owners were designed for such farmers and other users of such 3/4 and One ton Light Pickups NOT Hummers).

The chief problem with this is people will brag that their old car could go "faster" (This happened in the 1970s when the Big Three first down sized their engines to comply with the demand for better fuel economy AND the introduction of Pollution controls, both of which made cars "slower" then they had been in the 1960s). I remember reading about Police officers complaining that they brand new Police Cruisers could NOT keep up with the few remaining "Muscle Cars" of the 1960s when the Police chased such cars (one joke from the mid 1970s was that the Police Cruisers were all losing to an 1960 muscle car, and they then made a request for the then old 1970 paddy wagon with its big V-8 to chase the Muscle Car for the Paddy Wagon could keep up with the Muscle Car while the Police Cruisers of the late 1970s could not. These reports all disappeared in the 1980s for by then the old Muscle Cars were to old to operate. Thus the response to people who say if you ban cars with engines larger then 2.5 liter engines, the Police will have a difficult time catching criminals who operate older cars is to simply say, yes that will happen but over 5-10 years such older cars will be removed from the highways and the problem will thus disappear (as it did in the 1980s do to the older cars getting to old and costly to repair).

One last Comment, as to an exception of "Police and other emergency vehicles" unless such such users can use a 3/4 ton or one ton truck (And most ambulances fit into that category) the market for such products is to small. Right now, the Police use upgraded versions of cars sold to the General Public. The reason for this is the cost to produce the engines, bodies, transmissions and frames needed can be spread out over a lot of vehicles, not just Police Cars (And civilian operators of such Vehicles are willing to pay more then most police departments for the same products i.e. larger engines and transmissions, that cost to most to not only to make but to set up a factory to be made in). The Police lights, rear doors not able to be opened from the inside, the shotgun holder, the rear "Cage" are relatively minor additions compared to the Engine, Body, Transmission and frame. Basically if you restrict v-8 full size cars to the "Police" the Police will NOT buy them for the price would be beyond their budgets (i.e Police departments will have a choice either buy 10 2.5 liter Compact cars for the same price as full size police cruiser, do to the fact the cost to set up the factory for the engines, transmissions, body and frame will have to be spread over a very small number of vehicles, increasing the cost of such vehicles while the Compact car's cost to set up to make the Engine, Transmission, Body and Frame used in the Compact car can be spread out over 100 times the number of cars Police buy). Just a comment that any exception for Police will NOT be used, the cost to set up cars that are just to be used by the Police would price such cars out of the budget of most police departments.