A Tory councillor actually said that to me recently!
I know this has been discussed at various times on here.
Just need some simple arguments/useful links.
CityCyclingEdinburgh Forum » Resources
“Modern cars aren’t designed to go at 20mph” facts please
(37 posts)-
Posted 4 years ago #
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If they are talking about fuel economy (as opposed to how easy it is for the driver to control the speed), then:
Fuel economy is largely determined by driving style rather than maximum speed.
In an urban, stop-start environment, most of the fuel consumption occurs during acceleration. Accelerating from 0 to 20mph is going to use much less fuel than accelerating 0 to 30mph.
Also, 20mph allows drivers to anticipate further ahead, which smooths flow and in turn reduces stop-start.
For constant speed operation, fossil-fuelled vehicles have gears. The correct use of the gears is essential to achieve the best economy. If the driver is driving in the highest possible gear (without labouring the engine), there will be little difference in consumption between a constant 20mph and a constant 30mph.
Posted 4 years ago # -
This might not be exactly what you're after but it's related:
thanks to @Arrelcat for pointing me to that. It's more to debunk the myth that emissions go up when you drive at 20.
The reason it's related is that said councillor will probably tell you that you need to drive at higher revs in a lower gear in a modern car as the ECU doesn't manage loads at lower revs well (something I dispute happily on cruise control in my van at 20mph in 4th) - it is not true that higher revs mean higher emissions - I have posted previously ECU engine map load graphs which show the amount of fuel needed to be injected per stroke at certain loads. As you would expect you need more fuel per stroke in a higher gear with lower revs.
(Of course this pales into insignificance compared to the amount of fuel used to *reach* 30 but that's not part of this argument necessarily)
Posted 4 years ago # -
There is a good document from Ricardo available here:
The impact of 20 mph limits on carbon emissions and
air qualityPosted 4 years ago # -
Ha ha. algo beat me to it!
Posted 4 years ago # -
Also, very good foresight of Blackford Safe Routes to preserve that document, as it seems to have gone 404 on Ricardo's own site.
Posted 4 years ago # -
Thanks.
Not sure he really cares about emissions.
I don’t understand how, when people get to 20, they ‘know’ their car isn’t happy - so must be allowed to go faster.
Are there any ‘drive better at lower speed’ courses?
Posted 4 years ago # -
I must be really good at driving as I find it easy to keep to 20mph and my car seems to cope just fine.
Posted 4 years ago # -
"fossil-fuelled vehicles have gears"
I quite enjoy watching the queues that form behind me when I'm driving at 20 through the next town.
However the point about wheather cars are geared to do is fair.
I have small, big engined sports car, and if you drive it at 20 in 3rd it just stutters almost to the point of stalling, especially when cold. This seems to be quite common with petrol cars in general, as they don't have the same low down torque as diesels.When I drive at 20, in 2nd gear, which is most of the time, the car frankly becomes anti social because it's revving higher, and the exhaust starts to pop and gurgle loudly. It also uses quite a lot of petrol doing that.
Posted 4 years ago # -
Mrs Garto has no problem driving at 20mph in an automatic.
This is a valid counter argument to the councillor.
The car feels slow to the human but trhe car does not care
Posted 4 years ago # -
I did post on here a while back that, having conducted the experiment on several occasions, driving as economically as possible (my default style) my car uses 30% more fuel at 20 than at 30, so presumably 30% more emissions too at the lower speed.
I doubt whether anyone has tried to see if "economical" increase in speed between 20 and 30 offsets this result.
I think the point is that generalisations can't necessarily be made one way or the other. But we know there is a fair degree of pumping losses in a fourstroke motor at low revs.
Posted 4 years ago # -
“This seems to be quite common with petrol cars in general, as they don't have the same low down torque as diesels.
“When I drive at 20, in 2nd gear, which is most of the time, the car frankly becomes anti social because it's revving higher, and the exhaust starts to pop and gurgle loudly. It also uses quite a lot of petrol doing that.”
Interesting.
So, presumably, electric cars will be ok?
Posted 4 years ago # -
Never noticed any problems with my big, small-engined, petrol car going at 20mph in either 2nd or 3rd gear.
Posted 4 years ago # -
Yep, my 1.3 litre petrol has no problems in either 2nd or 3rd at 20mph.
Even in 3rd gear and low revs <1000rpm, you can put your foot down, and the electronics just deals with it*... not putting too much fuel into the engine to cause labouring and not providing any acceleration either...
*Amazing, eh? Technology is like that... just makes driving easy...
Posted 4 years ago # -
Presumably the automatic speed limiting technology coming in this decade will make it much easier to stick to the limit. A very temporary problem then.
Posted 4 years ago # -
Yep, yep, keep piling on. It's as if I said I never drove at 20 because of it - I *always* do.
It's just the gearing, my car is geared to do 60 in 2nd gear, so of course 20 in 3rd is going to be problematic. It's not that it can't do it, the car just lurches as if it's going to stall when it's cold.
I'd guess the 1.3's cylynders would be coming through the bonnet if it did 60 in 2nd. Different gearing...
(it's also 11 years old, so not as advanced electronics as newer cars, which is probably also why it only gets 27mpg)
Posted 4 years ago # -
Who's piling on? I don't think anyone's accused you of anything.
Posted 4 years ago # -
my car is geared to do 60 in 2nd gear
This is the unintended(?) consequence of manufacturers desperately trying to improve their 0 to 60 figures (by removing the need for an extra gear change to get to 60). 0 - 60mph figures being an entirely arbitrary construct, presumably coming originally from motoring journos and "back in the day" the fact that most cars could reach 60mph (and sometimes not 70mph).
PS. Am not trying to pile on, just providing some background
Posted 4 years ago # -
For a number of cars I'd say that 20mph does sit on the cusp of 2nd/3rd gear.
Modern tech can make life easier and blur the "accepted wisdom" that less RPMs (without labouring the engine) = more MPGs.
My current car goes into an "eco" mode between about 1500-2500 RPM where it will turn off 2 cylinders if the engine isn't labouring which means that often my fuel consumption at 20mph in 2nd gear is considerably lower (like 60-70mpg) than at the same speed in 3rd gear.Travelling at 20mph vs 30mph, as well as the often heralded safety aspects results in less and gentler breaking so you're getting less brake/tyre particles getting kicked up as well. It's also rather conveniently round about the speed a decent cyclist on a decent bike will cruise at on the flat so less close passes/overtakes! :)
I don't even think electric cars have a gearbox?
Posted 4 years ago # -
The idea of an electric car 'snapping' to certain speeds is quite attractive, a sort of intuitive pedal-pressure based speed limiter. Probably quite nice on the way 'up' the speedo but a bit hard to stop it being dangerous on the way 'down'...
Posted 4 years ago # -
I remember one BTL commenter in the chipwrapper complaining that the then proposed rollout of 20mph limits would be totally impractical because his car couldn't get up Craighouse Road if he stuck to 20mph. A number of other BTLers did point out the purpose of the sticky-up thing beside his left thigh, and the associated pedal adjacent to his left foot.
Posted 4 years ago # -
“Modern cars aren’t designed to go at 20mph”
If that is the case then modern cars are utterly unsuited to be used in cities, regardless of any posted speed limit.
Posted 4 years ago # -
I of course meant braking not breaking!
Posted 4 years ago # -
@Baldycyclists, it does sound like your car is a bit of an exception. Without knowing the model it sounds like it is more designed to go as fast as possible round a track without worrying about fuel consumption.
As you demonstrate it's entirely possible to drive such a car within the legal limit.
Posted 4 years ago # -
I always assumed noisy exhausts were a deliberate feature of these sorts of thing to try and draw attention to them, rather than an indication of them being asked to exceed their designed usage range.
If one was performing some sports in one of these sports car things, perhaps by accelerating from 0mph to 60mph (or even beyond), what gear would one be in at the point of passing momentarily through 20mph? Could one have such a vehicle retro-fitted with a manual choke to aid them in performing non-sporting practical tasks? My flatmate in first year had a Chevette, which was a long way from being practical and sensible yet had a manual choke.
Posted 4 years ago # -
I'm really replying so I can use my favourite motoring noun "swirl flaps".
Our 1.6L petrol people-carrier is old and tired. It has swirl flaps on the engine air intakes to make the air more turbulent at lower powers, which open more as load increases until at a certain power setting they're fully open.
Because car is old and tired, these flaps have been wired open (because they were getting stuck, and making an already underpowered car struggle to get over 30mph...). Therefore it now really doesn't like operating at certain low-power settings. 30mph is a bad notch, so I tend to sit at 27ish in fourth, rather than 30 in fifth. 20, as it turns out, is lovely in third.
I'm not sure what conclusion to draw with respect to the broader argument.
Swirl flaps.
Posted 4 years ago # -
@Darkerside Didn't know cars had those. Sounds kind of like intake cones/ramps for supersonic aircraft, but the other way round.
Posted 4 years ago # -
30 kmph is the normal speed limit on minor urban roads in both Japan and Germany. I find it hard to believe that the boffins at Nissan, Toyota, VW and BMW would design mass-market cars that can't operate efficiently at that speed. Although I'm willing to believe this isn't much of a consideration for AMG and the like.
I have had a good rake around for research which proves that driving at 20mph damages drive trains, but have not been able to find anything. I did find lots of advice to drivers not to thrash their cars, but that's a different matter.
Posted 4 years ago # -
The idea that modern vehicles can't be driven comfortably at a speed around or below 20mph is clearly [rubbish]. But (s)he's on to something. It does, often, feel like driving at the speed limit is difficult. This applies at 30mph 50mph 70mph.
The problem is that it feels awkward to drive at the speed limit on modern roads which have been engineered to allow much higher speeds (intentionally with features to allow speed faster than desired).
To put it more simply: modern city roads in the UK aren't designed to be driven at 20mph.
Faced with this as the problem it becomes blindingly obvious that the solution isn't to raise the speed limit (that starts to sound really silly), it's to change the street design to match the speed limit.
Posted 4 years ago # -
“it's to change the street design to match the speed limit.”
Quite so.
Posted 4 years ago #
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