How did the government manage to restrict the announcement last week to a tiny item in the Times? Did no other publications/broadcasters pick it up?
Reason given was NOT safety, but fuel conservation and pollution control.
|
Fuel conservation? How very noble and kind of the government to want to save me money.
I believe in Italy the limits been recently raised from 83mph to 91mph.
|
|
|
|
So why not reduce it to 40 or 30? Sorry - don't mean to give them ideas...
|
|
|
mybrainhurts wrote:
>
> How did the government manage to restrict the announcement
> last week to a tiny item in the Times? Did no other
> publications/broadcasters pick it up?
Was it a bad-news week? was there another big story going on?
didn't they just sack some spin doctoress for doing the same thing?
|
|
|
Fuel conservation!!!!
The most fuel efficient speed WAS about 56mph as we know and that is why speeds were set as they were. Can anyone tell me if this has now changed with more modern manufacturing methods?
|
There's a limit to how aerodynamic a car can be and still carry human beings in comfort. The problem is one of physical laws. The wind resistance curve steepens as speed increases. So the amount of effort it takes to travel at 100 mph is much more than twice the amount needed to travel at 50 mph; beyond a certain point for a given aerodynamic shape, each extra mph costs more in fuel than the last one. Which is why F1 cars need 800bhp to manage 230mph, but my 60bhp Citroen can manage (according to the manual) just over 100mph, despite the fact that it's much less aerodynamic. No getting away from this one folks: cars use less fuel at 70 than they do at 80, and always will. If you don't believe me, try riding a bicycle at twenty miles an hour for an hour. If you live long enough to complete the test, I guarantee you will feel more than twice as knackered as you would riding at ten miles an hour.
Chris
|
|
|
|
There is not a straightforward linear relationship between speed and fuel consumption. I doubt that reducing the speed limit to 30 would actually increase fuel consumption. My trusty Passat can do 43 mpg when driven at about 60 on the motorway (well, that is what the computer told me it was after a trip up the M5 and M6 - I was in no hurry, the motorway was empty and I wanted to see what I could get out of it by driving slowly). Yet, the instantaneous figure, rather than an averaged value, whilst doing 30 is more like 25. Not very scientific I know, rather an empirical observation.
One way to look at this is to consider the power needed to keep the car moving.
The power required follows an equation of the following form:
road load power = av + bv^2 + cv^3
Where v represents the velocity of the car, and a, b and c represent three different constants:
a comes mostly from the rolling resistance of the tyres, and friction in the car's components, like drag from the brake pads, or friction in the wheel bearings.
balso comes from friction in components, and from the rolling resistance in the yires. But it also comes from the power used by the various pumps in the car.
c comes mostly from things that affect aerodynamic drag like the frontal area, drag coefficient and density of the air.
These constants will be different for every car. But basically, if you double your speed, this equation says that you will increase the power required by much more than double.
Now, the equation suggests that a stationary car has a zero power requirement and thus infinite mpg. This is of course not the case as fuel is still required to run the engine. Since fuel is being used to run the engine but the car is stationery the mpg is 0.
If the car is moving at 1 mph, the car uses only a tiny bit more fuel, because the road load is very small at 1 mph. At this speed the car uses about the same amount of fuel, but it went 1 mile in an hour. This represents a dramatic increase in mileage. Now if the car goes 2 mph, again it uses only a tiny bit more fuel, but goes twice as far.
In effect the efficiency of the engine is improving. It uses a fixed amount of fuel to power itself and a variable amount of fuel depending on the power required to keep the car going at a given speed. So in terms of fuel used per mile, the faster the car goes, the better is made of that fixed amount of fuel required.
This trend continues to a point as a result of the road load equation. Once the speed gets up into the 40 mph range each 1 mph increase in speed represents a significant increase in power required. Eventually, the power required increases more than the efficiency of the engine improves. If you put some numbers into the equation you can see that the increae in power required to go from 50 to 51 mph is much greater than to go from 2 to 3 mph.
In general, because of things like coefficient of drag, frontal area and weight smaller, lighter, more aerodynamic cars will get their best mpg at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mpg at lower speeds.
|
|
|
Maybe the much-maligned EU should sort out some speed limit harmonisation at more realistic mainland European levels.
Oh and diesel duty harmonisation wouldn't come amiss either.
|
|
|
I was under the impression that manufactures designed in ultimate fuel efficiency at consistent speeds of 56 and 75 mph to produce the best results for official mpg figures and can mean more fuel is used at 70 than 75 in some cases. Which also begs the question why the 75 mph figure is used in the first place when we are restricted to 70!
I missed the announcement, can anyone suggest a reference on the web?
Rob S
|
|
|
|
Fuel conservation? We have more of the stuff than we know what to do with, and long before it runs out we will have discovered alternative means of propulsion. Just a case of the government not having the stomach for a fight with the safety lobby, "They're murdering our children, destroying life as we know it, discriminating against one legged skateborders blah blah blah etc etc"
|
|
I admire your optimism. I doubt that I'll live long enough to see it fulfilled or otherwise. It's my kids and their descendants I care about. Being 'swamped' with oil, or any other finite resource, doesn't absolve us from good stewardship whilst on this planet.
|
|
|
|
