Gary G Posted September 22, 2002 Share Posted September 22, 2002 Not 7 related, but interested to know people's views: Driving my tin-top yesterday, getting close to an empty tank of fuel I started to drive in a way that minimises fuel consumption. Many people say that the most fuel efficient speed to drive at is 56mph. I think this is based on manufacturer figures being quoted in 3 categories: Urban, Extra-Urban, and combined. Normally the best figure is the one where the car is travelling at 56mph. But is 56mph the most fuel efficient speed? I find it hard to believe that this speed can apply to all cars, since they all have different aerodynamics and friction. What do you reckon? C7 GAR Oh Yes Yes Yes Yes Link to comment Share on other sites More sharing options...
Dirty Den Posted September 22, 2002 Share Posted September 22, 2002 My tintop has a hair dryer for an engine. I find that I get the best fuel economy on journeys if I try and keep to 50-60 mph. If I go any faster than that, not only does it sound like a cat being drowned, but the fuel economy does worsen a little. Having said that, I get an average of around 300 miles to £20 worth of unleaded. Den 😬 - Self portrait - still unable to remove the smile! Edited by - Dirty Den on 22 Sep 2002 17:44:21 Link to comment Share on other sites More sharing options...
Mark H Posted September 22, 2002 Share Posted September 22, 2002 the slower you go the less fuel you burn, basicly still wearing flares 😬 Link to comment Share on other sites More sharing options...
scooby dooby doo Posted September 22, 2002 Share Posted September 22, 2002 its not that simple... at idle you are burning fuel but going nowhere. at 1mph most of the petrol is going to keep the engine turning over and a tiny bit into moving the car. so not very fuel efficient. at 70 mph you may not be revving very high as you have a nive long top gear but the engine is producing a lot of power to overcome wind resistance. so not very fuel efficient. so the optimum must be in the middle somewhere. assumin its a windless day and the road is flat etc i'd guess (gut feeling, no more) its somewhere between 30 and 50.... HOOPY R706KGU See you at Cadwell Link to comment Share on other sites More sharing options...
F355GTS Posted September 22, 2002 Share Posted September 22, 2002 the 56MPH equates to 90kph and is a point set where MPG is required to be reported by manufacturers. Every car will be different Mark Link to comment Share on other sites More sharing options...
C7JFM Posted September 22, 2002 Share Posted September 22, 2002 Is it better to gun it to 56 or to sloooowly amble to 56 ??? Link to comment Share on other sites More sharing options...
Mark H Posted September 22, 2002 Share Posted September 22, 2002 i did say basic , still wearing flares 😬 Link to comment Share on other sites More sharing options...
Mark H Posted September 22, 2002 Share Posted September 22, 2002 At idle you are burning fuel BUT at zero load so less is burnt than when moving but if in top gear at say 2000 rpm you maybe traveling at 30 mph and using less fuel than when traveling at 30 mph in first simply because you would need more revs = more fuel but then if you take into account frictional loses and aerodynamics it starts to get complicated( well too complecated for me a simple mechanic to explain ) Basically if you travell in top gear at 40 mph as opposed to 50 mph you will travel further,for a given amount of fuel, still wearing flares 😬 Edited by - Marc Hicks on 22 Sep 2002 19:46:10 Edited by - Marc Hicks on 22 Sep 2002 19:46:49 Link to comment Share on other sites More sharing options...
excess baggage Posted September 23, 2002 Share Posted September 23, 2002 If you're in a 7 who gives a ****? Link to comment Share on other sites More sharing options...
tonym Posted September 23, 2002 Share Posted September 23, 2002 I would have thought that a moving vehicle would be most fuel efficient at or around peak torque ? Link to comment Share on other sites More sharing options...
scooby dooby doo Posted September 23, 2002 Share Posted September 23, 2002 peak torque may be most effiicent from the engine's point of view but aerodynamics and drivetrain losses get in the way.... HOOPY R706KGU what's a 'hood' 🤔 😬 Link to comment Share on other sites More sharing options...
Peter Carmichael Posted September 23, 2002 Share Posted September 23, 2002 Peak torque is at the peak of volumetric efficiency, which is often mistaken for meaning "efficiency". You can only operate continuously at peak torque (meaning foot flat on the floor) if the road loads match the engine output - which you might think about either as the torque or power that the engine produces. ie. you never cruise at peak torque. Now consider how sophisticated a device a throttle butterfly is. S'right, it isn't. It reduces the power output of the engine by preventing air getting in. Peak torque was that point where the cylinder was most filled with fuel/air mixture. When the throttle is at anything other than wide open you aren't at peak torque. Indeed, you aren't at peak anything. You are somewhere within the operating envelope of the engine. Consider the two problems: 1. How to cruise at 50mph at best efficiency. 2. What speed to cruise at for best fuel efficiency. In the first situation, you have a choice of gears in which to deliver the same power demand. If you choose a low gear, the engine revs highly but the throttle is almost completely closed. If you choose a high gear, the engine chugs along and you have to keep the throttle comparatively wide open - it turns out that this is the more fuel efficient way of going about things. In the second situation, you also have to consider that aerodynamic losses increase as speed increases. This turns out to mean that slower is better in as high a gear as possible. The only exception to this is if you go so slowly that the engine is forced to labour in which case its efficiency falls away markedly. It is however a fairly pointless exercise as getting everywhere at 10mph in fourth gear would soon get you arrested for driving dangerously. This turns out to be reasonably clear from the thermodynamics point of view if you review the P-V (indicator) diagrams and is, as stated earlier, down to avoiding pumping losses. Link to comment Share on other sites More sharing options...
scooby dooby doo Posted September 23, 2002 Share Posted September 23, 2002 by "may" i meant - i don't know, not i think it is... not sure i was very clear on that. but it comes down to you reckoning that as low a speed as you can magage in a direct (ie 1:1) gear without labouring. so it'll be 15mph in 6th or similar... hmmm... anybody bored enough to try this 🤔 HOOPY R706KGU what's a 'hood' 🤔 😬 Link to comment Share on other sites More sharing options...
Mole Posted September 23, 2002 Share Posted September 23, 2002 Peter - I think you lost me somewhere in the first paragraph ❗. According to the published performance stats, peak torque on my engine is at 3000 rpm - this is far from "foot flat on the floor" - What have I misunderstood? SV 52 CAT - the Mole is flying Link to comment Share on other sites More sharing options...
Peter Carmichael Posted September 23, 2002 Share Posted September 23, 2002 In your case, peak torque is at 3000rpm with your foot flat on the floor. Unsurprisingly this causes your car to accelerate - therefore you can never cruise at peak torque. If you operate the engine at 3000rpm but only part throttle you are not operating the engine at peak torque. The "peak" is a characteristic of the wide open throttle output of the engine. Link to comment Share on other sites More sharing options...
Graham Sewell Posted September 23, 2002 Share Posted September 23, 2002 The peak torque at 3000rpm is with a wide open throttle, it just stays at 3000rpm for a couple of milliseconds on its way up! Either that, or the engine is being loaded by a brake so that it cannot increase in speed despite the wide open throttle. As an aside, during the petrol blockade a couple of years ago, by changing up at 2000rpm in my tin top (revs dropped to 1300rpm) i managed to get > 10m/l around town. It is the most boring way to drive, but it meant a theoretical range just under 500 miles on a 11 gallon tank! Low tech luddite - xflow and proud! Link to comment Share on other sites More sharing options...
Graham Sewell Posted September 23, 2002 Share Posted September 23, 2002 Damn - double post Edited by - Graham Sewell on 23 Sep 2002 12:29:29 Link to comment Share on other sites More sharing options...
Mole Posted September 23, 2002 Share Posted September 23, 2002 Peter and Graham - thanks, now I understand SV 52 CAT - the Mole is flying Link to comment Share on other sites More sharing options...
Gary G Posted September 23, 2002 Author Share Posted September 23, 2002 So do we have an answer to the question: Is 56mph he most fuel efficient speed for ALL cars? I doubt it is, but can some of you techies confirm this? C7 GAR Oh Yes Yes Yes Yes Link to comment Share on other sites More sharing options...
Peter Carmichael Posted September 23, 2002 Share Posted September 23, 2002 No it isn't. Link to comment Share on other sites More sharing options...
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