I'm not bothered about traffic light grand prixs so am not interested in gearing for them, but it would be nice if they geared cars for maximim economy at 70/75mph, that way if you want to go faster then fine, but those of us who can't be bothered mixing it in the outside lane could cruise along knowing that we have the best possible fuel consumption!
Oh, and whilst we are on gearing, a gear ratio that suits the engine/car for running at a constant 30mph so you are not having to labour it in too high a gear or revving too high in a lower one...
Please!
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Maximum economy for cars is usually around 30-40mph in top gear.
Wind resistance rapidly becomes the major factor, rather than engine efficiency.
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Yes, I know, but I was thinking of max economy at 70 (motorway cruising speed), as I said correct gearing for 30mph running would also be nice... gearing for 70 at 2000rpm would be ideal for my car, but instead its geared at 2500/70, it could do with a sixth gear, perhaps...
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Maximum economy for cars is usually around 30-40mph in top gear.
My car wont run in 6th gear below about 60mph, 4th is ideal for 30mph.
It is at 2,000rpm at 70mph in 6th, almost on the max torque point, so about optimum for economy.
Edited by Old Navy on 13/07/2009 at 09:41
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Maximum economy for cars is usually around 30-40mph in top gear. >>
I thought the most ecomonical speed was 56mph as that all the car manafacturers quote best figures on?
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The old standards for fuel economy were 'urban cycle', constant 56mph, and constant 75mph. These were changed around 15-20yrs ago to the current urban/extra-urban/combined.
The best mpg from the old standards would always be the 56mph test, but as I mentioned before the very best fuel economy is around 30-40mph as the engine is just able to run in top gear without undue labouring.
I recall a test in WhatCar? about 2 yrs ago with 6 or 7 different cars at steady speeds from 30 to 90. Every car's fuel economy dropped at each 10mph interval, with the worst being the least aerodynamic vehicles or those with relatively low gearing for high speed (I think Citroen C1 and Qashqai showed the biggest falls at 70+ mph).
tinyurl.com/mpgmyth
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I'm not bothered about traffic light grand prixs so am not interested in gearing for them but it would be nice if they geared cars for maximim economy at 70/75mph that way if you want to go faster then fine but those of us who can't be bothered mixing it in the outside lane could cruise along knowing that we have the best possible fuel consumption!
It's comforting to hear some sence for a change someone wanting transport and not motor sport!
There are some of us about, content with following at a reasonable distance at an economical speed
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Well said Farmer Boy. A voice of sanity.
I am an enthusiast for cars because I admire good engineering, not because I fantasize that I am Jenson Button!
Also, good points made by b308 up-thread. The problem with setting up the gears for maximum efficiency at certain speeds is compounded by the fact that manufacturers just offer a standard setup for all of their cars, whereas different countries have adopted slightly different legal speed limits.
You would expect German manufacturers to set their cars up for German roads, naturally.
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You would expect German manufacturers to set their cars up for German roads naturally.
Agree with that, every good (larger engined) German car i've driven has had better acceleration at progressively higher speeds, sometimes at the expense of initial take off.
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Suppose the manufacturer set up the gearing for best econonmy at 70mph. That in fact would be a pretty hig gear as it would mean that you would achieve 70mph at full throttle on a level road with no headwind. Any incline whatsoever or any headwind whatsoever would mean changeing down at least one gear so top gear would in fact only be of any use in ideal conditions. With top being so high the spaceing of the other gears would would of course be wider than ideal. Therfore, the ratio of the top gear will have to be a compromise between ideal for economy and allowing some leeway for the real world.
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I might be reading you wrong, L, but if you set up the gearing so at 70 the engine was running at its most economical it would not be running at max revs - in my car it would be fine for it to be geared at 2000rpm at 70, its got plenty of torque down to 1500rpm so only the really steep hills on the Autobahns would cause me to have to change down.
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>>it would not be running at max revs
nobody said it would
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Regarding fuel economy, my parents' BMW E39 520i Touring (1.6 tonnes approx, 2171cc, 168bhp) is actually at its happiest and most economical (around 40MPG) at 80-90mph... now, if only all cars were that good.
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Wow - never knew Bavaria had defied the laws of physics.
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>>it would not be running at max revs nobody said it would
"it would mean that you would achieve 70mph at full throttle on a level road with no headwind"
I'm am obviously reading that the wrong way, then?
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>>I'm am obviously reading that the wrong way, then?
Bluntly, yes!
The throttle position isn't directly linked to engine speed.
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The way I read it (obviously incorrectly) was that at 70mph (I assumed steady 70, not accelerating up to 70 as my post did not talk about accelerating, just travelling at a steady 70) it would run at full throttle, I feel my interpretation of the wording is just as acceptable as yours, though I now see where the poster was coming from!
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I feel my interpretation of the wording is just as acceptable as yours
You may feel what you like!
;-)
An engine tuned for maximum economy at 70 mph would not be revving anywhere near maximum revs, although it migh be operating at nearly wide open throttle. I'm afraid your interpretation makes no mechanical sense at all. Sorry.
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Now I'm getting a little lost... and have probably found why I'm not with you! Are you saying that if my car was operating at peak torque (Which would also be best rpm taking into account any undulations in the road - say 2200rpm) at 70 it would also have max throttle opening? I've always thought that max throttle opening would be "foot to the floor", but thats obviously not the case from your posts...
Any chance you could put an explanation in layman's terms for me, NC? ;)
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>>Are you saying that if my car was operating at peak torque (Which would also be best rpm taking into account any undulations in the road - say 2200rpm) at 70 it would also have max throttle opening?
Yes - but, obviously as all car engines are vastly oversized, it would soon accelerate beyond 70.
>>I've always thought that max throttle opening would be "foot to the floor", but thats obviously not the case from your posts...
Yes, there's no conflict here.
If the engine were set up to just be able to do 70, as per lotusexige's post, with your foot nearly flat to the floor, it would be quite economical. It wouldn't necessarily be revving like mad. However, if you came to a hill, or faced a headwind, you wouldn't be able to maintain 70mph.
If you can think graphically,...
Imagine a graph with road speed in mph along the x axis, and power in BHP running up the y axis.
If you calculate the total drag power for the car and driveline at each road speed, you'll get a curve which starts off at a small power at lower road speeds, rising quite rapidly at higer road speeds as aerodynamic drag begins to dominate.
If now you super impose the engine's power curve onto the graph (imagine that you have drawn it onto a transparent sheet which you overlay and can slide left and right, but cannot move up and down). If you slide the engine's power curve to the left of the graph, that's a lower gear, and the difference in height between the two graphs means that there is lots of available power to accelerate the car.
Remember that the drag power curve is concave upwards, and the engine's power curve is concave downwards.
As you slide the engine power curve to the right, effectively mimicing a higher gear, you'll reach a point where the graphs will cross - somewhere just beyond peak engine power).
By arranging the crossing point to fall at the peak engine power, the road speed which corresponds to this particular crossing point is the maximum speed the car could ever theoretically acheive with perfectly chosen gearing.
If you continue to slide the engine power graph to the right, the rev range where the engine power is above the road load power becomes narrower, and the height difference between the two curves, the available power becomes less. This is the regime which Lotusexige was describing.
Eventually, continuing to slide the engine power curve to the right, you'll reach a point where the engine power is always below the road load power, and the vehicle could never drive in that gear - it would stall from any engine speed with any throttle opening.
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70mph at full throttle on a level road with no headwind"
Actually in an engine tuned for economy, full throttle is very economical. In the days when British cars in particular were quite low-geared, Mobil Economy Run contenders used to drive in a totally horrible way, banging the car into top at some very low speed and driving at full throttle until it reached 50 or so, then turning the engine off and coasting back down to the lowest speed, full throttle again to 50, and so on all the way to John O'Groats...
The lowest circle of the Inferno, my dears... 95 mpg with a carb notwithstanding...
Edited by Lud on 14/07/2009 at 21:07
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>>full throttle is very economical
Yes, because energy is not being wasted dragging air across a closed throttle.
If you look at an engine performance map, you'll see that in order to get the most power per unit fuel, you run the engine at near the maximum torque speed, at about 3/4 of full load.
Herein is one of the big advantages of the traditional diesel engine - there's no throttle, and so, part load efficiency is not compromised.
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wasted dragging air across a closed throttle.
Just so NC... yet in the carb days high inlet vacuum (which you got on lowish part throttle rather than wide open) used to equate to good fuel consumption if the carb was any good.
I know you think carbs are an inefficient old way of metering air-fuel mixture, as of course they are, but you have to love the things like Harrison chronometers... of course I mean well-designed ones for their purpose, not horrid Ford or GM model-specific devices which sometimes worked and sometimes didn't...
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>>yet in the carb days
Nothing has changed.
A wide open throttle gives good power per unit fuel, but, as most engines were and still are grossly oversized, yo get the best out of the vehicle at near closed throttle, hence the desire for high manifold vacuum.
Yes, I'm not mad keen on carbs - a modern fuel injection system can do far more.
That's not say I don't admire the admirable simplicity of something like a nice SU with a side sprung needle (non of the old jet centreing nonsense!). However, anyone who has ever struggled with something truly horrid like a Solex 4A1 won't be mad keen to repeat the experience.
The 4A1 is a true villain of a carb, it makes carbs like the Ford VV look rather tame!
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I know you think carbs are an inefficient old way of metering air-fuel mixture as of course they are but you have to love the things like Harrison chronometers... of course I mean well-designed ones for their purpose not horrid Ford or GM model-specific devices which sometimes worked and sometimes didn't...
A nice DCOE Weber, a beautifly enginered instrument designed to give the wrong mixture under all conditions. Don't forget the lovely sound.
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Oh, and whilst we are on gearing, a gear ratio that suits the engine/car for running at a constant 30mph so you are not having to labour it in too high a gear or revving too high in a lower one.
that would be 3rd gear on my Octavia :-)
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Bravo, CGN! I've yet to drive a car that wasn't happy at 30 in third. But that's an argument for another thread, I think.
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It is often mooted that town driving in 3rd is probably most sensible - you are more aware of the engine speed and are less likely to go over 30mpg. Also many modern cars won't pull much of a hill at 30 in 4th+.
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Bravo CGN! I've yet to drive a car that wasn't happy at 30 in third. But that's an argument for another thread I think.
If your ever in my area you can try mine, subject to insurance. It is a bit on the extreme side though.
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