My view is that I don't believe it will.
Yes, a constant speed is almost always the most efficient way of using fuel, at the right speed that is, but with the high volume of vehicles on the roads, will it actually work?

I suspect all it will do is bring down the average speed of those that usually drive at 80 plus, to a slightly lower level. Does driving faster actually increase the volume of pollutants expelled? After all, the fuel is only being burnt at a faster rate at higher speed, so surely a litre burnt in say, an hour as opposed to thirty minutes will still convert to the same amount output, only over an extended period of time?

A constant speed would be nice rather than the usual stop start

I imagine a car makes more noise pollution travelling at a higher speed, so cut the speed and you reduce pollution, albeit marginally.

Toyota Prius is more economical at 80 than it is at 60.

They were talking about raising the speed limit to 80. We are already one of the oldest, slowest nations in Europe. The 70 mph limit was introduced in like the 1950s or 1960s thanks to the Daily Mail kicking up a fuss.

It doesn't matter to me as I would do 70 regardless as I'm sure other people would do 80 or 90 regardless still as well.

My biggest bugbear is the IDIOTS (waits for Exhausted to complain) who come past you and pull into your lane ahead of you, only to then slow down once they are infront of you causing you to either have to slow down yourself and wait for a gap in traffic or otherwise overtime them.
That or people who come past you and then slow down whilst along side you doing the same speed, meaning that you need to slow down or floor it to be able to overtake the lorry you're approaching.

If I'm sat on cruise control at 70 I d@mn well know it's not me!! ****ing morons honestly.

Changing the speed limit would not allieviate these problems, and I don't think the Government cares in general about polution as they have configured most traffic lights to go red when you are approaching so you have to stop and slow down (thus wasting fuel by tuning speed into heating your wheelarches). The best thing that can be done is make cruise control compulsory and force people to use it.

That Prius factoid from the Autocar review seems to have been discredited.

It's entirely possible that powerful reasonably sleek cars would be more economical at 80mph than at 60mph, but not likely for a Prius or for your typical family car either.

The faster a car goes the more work it has to do against wind resistance. Of course the faster a car goes the less time it takes to get from A to B, but even so, the faster you go for a given journey the more fuel you'll use overcoming wind resistance.

But that's not the whole story. The economy of the engine varies strongly with its power output, becoming more economical with increasing power output until it reaches a maximum economy and then becoming less economical again as the power increases.

So the overall economy of a car depends both on the economy characteristics of its engine and on the economy characteristics of its bodywork, and so it's possible for the economy of a car with a big motor in a sleek shape to be better at 80mph than at 60mph because, despite using more energy to overcome wind resistance at 80mph than at 60mph, the economy of the motor is worse still at the power needed to propel the car at a stead 60mph than at a steady 80mph. But that seems unlikely for a Prius.


But anyroad, the proposed speed restriction is to reduce local pollution from what I assume is things like NO₂ which is much worse from Diesels than it is from Petrol cars, and again I'm assuming NO₂ emission is significantly better at 60mph than at 70mph, though I'd like to see the evidence of that first.

The most efficient speed of an engine is at it's peak torque RPM - the point at which the engine needs makes the most efficient use of it's power.
This is very car dependant but generally around 50-60mph in 5th or 6th gear.
My TDI was pretty much equal at 60 or 80, not really much in it. Where as my GTI is much more economical at 70 than it is at 80.
So it is very dependant.

If the Prius article was discredited then that's fine haha. Irrespective of my hate of the cars and the people who buy them, thinking they do the world a favour. I could get a genuine 65 to the gallon out of my old TDI, I doubt you'd get that from a Prius the way I drove my TDI.
And my Golf was not shipped twice around the world and heavily mined for the batteries.. Although I think they've changed to Li-ion now away from the Nickel and Cadmium ones they used in the earlier cars?

To travel faster through air, you have to add proportionately more horse power. Aerodynamics, gear ratios and engine efficiency all play a part though.



If we are to have an electrical replacement for the internal combustion engine, then cars like these will be considered the stepping stones to that goal.

A mere drop in the ocean and kowtowing to EU directive just to tick a few boxes.
What with the population continually growing and proposed 3rd runway at Heathrow etc. etc. what is the point?
If you are going to reduce pollution then you have to take far more drastic measures than dropping the speed limit by 10mph!

... and owning a dog is far worse for your carbon footprint than a 4x4 (including CO2 produced feeding the animals for their feed).

Don't hear many of these "treehuggers" calling for a ban or limit on dog ownership - perhaps because they are dog owners!

I didn't know that, but I can just about see that it's right. Power goes like the product of the torque and the revs, and the efficiency goes like the ratio of the power and the volume rate of fuel use, so if the engine is turning at the revs at which it produces its maximum torque then it seems reasonable that it would produce its maximum efficiency at that revs at some lesser torque, though I can see the complexities of the internal combustion conspiring to move that maximum efficiency revs a little.

Point is, an engine is only running at its best efficiency when it's running at a specific revs and producing a specific power, and the car it's in is only running at its maximum efficiency if it's driving at the speed and in the appropriate gear to offer the engine its best-efficiency load at its best efficiency revs. All very difficult to do in practice.

What would potentially do the job much better is to drive the wheels with electric motors from a small bank of rapid charge/discharge batteries which are continually topped up by an engine running all the time at optimum revs under optimum load, with the batteries charging when the car needs less power than is available, and the batteries discharging when the peddle hits the metal. You'd have the extra weight and inefficiency of the electric motors, but it would be interesting to know how much more efficient the engine could be. You'd also get regenerative breaking of course and that might be worth quite a bit round town.

Is this how hybrids work, or are the wheels driven directly by the hybrid's internal combustion engine?

Honda have already done it with the FCX Clarity.
Hydrogen fuel cell.
Electric cars are NOT an answer. They are not capable of our day to day way of life.

I seem to remember that Stan Green proposed a ban on dog ownership in the letters page one time. Speaking as a tree-hugger, it's not an idea I like.

Both technologies face big problems. Hydrogen fuel has no infrastructure to sell to the consumer (that'll be expensive to roll out), and electric cars are slow to charge.

Hydrogen DOES have infrastructure. There are existing petrol filling stations along with LPG stations. We can utilise 90% of the existing infastructure there, would just be pumps etc.

Quite right, for both alternatives refuel infrastructure is essential. Hydrogen wouldn't be particularly difficult to roll out, in many respects just like gas. Ironically, one of the big barriers would be public acceptance, so far they seem concerned about hydrogen which is perceived as more dangerous than petrol. In reality it isn't. For electric, it's surprises me that the manufacturers haven't got together and come up with common battery sizes and connectors - enabling a vehicle simply to pull up, drop it's discharged batteries and pull up a recharged set. Probably a quicker process than refuelling with petrol. Perhaps laying the groundwork for that could be an HMG / Euro initiative.

Yes, but what are aeroplanes going to run on, and boats, and lorries??

Interesting idea - especially as a big disadvantage of batteries is the cost of replacing them when they fail. If it was possible to 'refuel' by swapping batteries it would negate this cost (or absorb it into the cost of the battery swap). But from what little I've seen of electric cars a battery swap would be a complicated process requiring some sort of mechanism to carry the very weighty battery packs from filling station to car and vice versa. I can't see it ever being as quick or simple as refueling with a liquid or gas.

Back to the 60mph limit proposal - essentially an expansion of the limits already in place on the M1 - I understood these were part time limits aimed at reducing congestion. If it worked then there would be far more emissions improvements from getting rid of jams rather than any reduction in the emissions vis-a-vis travelling at 60 as opposed to 70mph.

Not until they are empty of petrol.

Big, big farts!

Simon sorry I missed this post.
About your first bit, generally this is taken to be top gear at around 1800-2200 RPM. As you say in theory may be one thing but in practise is quite another. So as a rule we say around 40-60mph in top gear again this can be very dependent on what car you have.

Hybrids work by having a normal engine and some batteries and electric motors. The car is capable of running on the electric motors/batteries for a SHORT period of time (normally less than 15 miles), most of the time the car is a FUEL powered vehicle. The batteries are charged by the engine when it's also powering the car, and some have regenerative braking and what not as well.

I have not owned one but an ex-work colleague had one and he said he got around 450-500 miles to a tank and the trip computer was reporting an average long term of around 48 to the gallon - So not that impressive at all, especially considering the fact Prius' are NOT a cheap car and neither are they particularly nice to drive or in any way interesting.

Problem with rapid charge/discharge is that their lifespan is usually very short.
In the government emissions test the hybrid cars ALWAYS do well because usually the electric motor is used for the majority of the test.. the 918 Spyder is listed as doing 98mpg for crying out loud .. it's a 600bhp petrol engined supercar (+ the power from the electric motors...)