OK. I'll try to explain one last time. Let's leave the confusing area of 30 and 20mph zones and talk about 50mph. The issue is exactly the same just at a different speed. It doesn't matter if your car is manual or automatic. It's the same issue. I'm not going to use arithmetic, so don't sweat it.
So, you're pootling along at 50mph in top gear on a whiff of throttle at say 2,000rpm. You cover one mile on a level road. You use a certain amount of fuel to cover that mile. OK?
You drop down a gear [it doesn't matter why in this explanation]. So you are now going to do the next mile under exactly the same conditions, but in a lower gear. To stay at 50mph your car engine will spin at higher revolutions and use slightly more fuel to keep it there [otherwise it will slow down slightly]. So, higher revs, more fuel, same speed, same time to cover the distance.
If you decide to stay at the same engine revolutions as before, when you were in top gear, in order to consume fuel at the same rate, the car will be travelling at a lower speed because it is in a lower gear. Therefore lower speed, same revs, longer time to cover the distance, more fuel consumed [because you're taking longer to cover the distance].
So that explains why cars consume more fuel, and consequently pollute more, in lower gears, given the same parameters. [the same car will always consume more fuel over the same given distance in a lower gear, even at exactly the same throttle opening - it will therefore pollute more]
Modern cars have taller gear ratio sets and more gears in the set [is that too techy?] than cars of yesteryear. Generally, they develop their peak torque figures higher up the rev range than they used to back then, even diesels due to developments from such people as Markus Liebherr [who he?] In general this makes them more relaxing to drive overall, but with a spirited performance if required, without totally drinking the amber flammable stuff. But it does mean that engines don't like running smoothly or efficiently in top gear at anything less than 30mph. There are exceptions, as mentioned before.
So, in order to accommodate driving smoothly at less than 30mph, say in a 20mph zone, the driver generally has to drop to a lower gear. For the same reasons given above, that results in higher fuel consumption and consequently more pollution.
But there is an additional pollutive element here. Every time the car changes gear [up or down] and takes up the load of the new gear, there is slightly more pollution produced. And every time a car accelerates to a higher speed, fuel is consumed at a higher rate compared to a normal cruising throttle. So more pollution is caused. And, because the car is accelerating, the fuel mixture is temporarily made richer, so even more pollution. You've all seen badly serviced diesels belch a plume of pollutents every time one accelerates? That's an extreme example of the mixture being temporarily rich due to incomplete fuel burning, but even a well tuned car will produce more unburnt fuel when accelerating than when on a cruising throttle.
And fixed gear automatics behave in exactly the same way. When Beddy's 7 speed automatic is in 5th gear, he's consuming more fuel and polluting more, given the same distance. Constantly Variable Transmissions [CVT] do the same thing but they effectively have infinite gears. They are still subject to high points of gearing or low, so the case still holds.
You’re missing the crucial point that at 20 mph it takes longer to get anywhere than at higher speeds, so overall your fuel consumption goes up.As you mentioned me I put your theory to the test this morning...........I changed my dashboard settings so I could read what my fuel rate is as I'm driving. The limits around the 20mph zone are as follows 40mph leading to 30mph leading to 20mph approximately .5 of a mile leading to 30mph In one direction. The 20mph zone is slightly down hill at my first attempt.........I dropped from exactly 30 to 20 my revs dropped and my fuel consumption dropped too. Not by much I grant you but it did drop.
The revs did go up slightly as I Exited the 20 zone as I reached 30...but again not by much. (My normal driving stance) no fast acceleration. On my return journey it was 30 to 20 to 30 my revs did pick up as I went through the 20 zone as it was going slightly up hill. Yes my fuel rate did go up slightly but not by very much. So perhaps it does depend on the gradient as to whether dropping to 20 will use more fuel?. So I tried another experiment..........Near us is a country lane I drove to that and from 30 dropped to 20 (Along the flat bit nobody behind) my fuel gauge flickered down and back to the same as the 30. So I don't know if that supports your argument or disproves it? Pollution........I haven't a clue..........
Edit............. my gear ration did not change by the way although had accelerated faster it might have done.
You’re missing the crucial point that at 20 mph it takes longer to get anywhere than at higher speeds, so overall your fuel consumption goes up.
Humans should have just drawn the line at 900cc Minis, the environment would have been saved.
As you inferred... Only in America.Here's a weird story from the US...the land of weird stories. A 5 month pregnant woman attacked another woman in Alabama and got shot (killing her baby). The woman who fired the gun was let off because of a plea of self defence (without knowing the full circumstances that could be possible), but the woman who lost her baby has now been charged with murder for being reckless whilst pregnant. She is said to be totally responsible for the death of the baby. Stupid and reckless certainly, but murder! I don't think it is entirely unconnected with her being black, female and this being in Alabama (and I am not a person who jumps to a conclusion about racism). She is probably not a good person and may have been a terrible mother, but this alone isn't necessarily a crime.
As Chilco says, you missed the essential point. But there are several contributing factors. The original point was that modern engines don't like running at 20mph in top gear, whereas they are usually fine at 30mph. This means that to have them running smoothly again means dropping down a gear. More fuel consumption over the same distance - more pollution. That's the simple fact.As you mentioned me I put your theory to the test this morning...........I changed my dashboard settings so I could read what my fuel rate is as I'm driving. The limits around the 20mph zone are as follows 40mph leading to 30mph leading to 20mph approximately .5 of a mile leading to 30mph In one direction. The 20mph zone is slightly down hill at my first attempt.........I dropped from exactly 30 to 20 my revs dropped and my fuel consumption dropped too. Not by much I grant you but it did drop.
The revs did go up slightly as I Exited the 20 zone as I reached 30...but again not by much. (My normal driving stance) no fast acceleration. On my return journey it was 30 to 20 to 30 my revs did pick up as I went through the 20 zone as it was going slightly up hill. Yes my fuel rate did go up slightly but not by very much. So perhaps it does depend on the gradient as to whether dropping to 20 will use more fuel?. So I tried another experiment..........Near us is a country lane I drove to that and from 30 dropped to 20 (Along the flat bit nobody behind) my fuel gauge flickered down and back to the same as the 30. So I don't know if that supports your argument or disproves it? Pollution........I haven't a clue..........
Edit............. my gear ration did not change by the way although had accelerated faster it might have done.
But surprisingly no better for the environment, according to the odd test or two. Me? I'm halfway through my last purchase - a bookYou must log in or register to see images




