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I’m sure some of you remember that driving at a constant 55mph was considered optimum efficiency, is this still the case with modern, fuel injected engines ( I’m talking about petrol cars) , or is there some new more efficient speed, or with computers in cars does this no longer apply, I also remember looking at the tailpipe after a longish run to see if it was a creamy brown colour indicating that the engine was running effectively, as I owned several BL cars, I couldn’t do this as they all seemed to burn a bit of oil, the only one I remember seeing this on was an alfasud I once had, other than that, I can’t remember another car I had that did it...
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For my last few petrol cars it seems to have been about 62mph, which is funnily enough 100kph. The Kia Sportage 1.6 gdi I have had for the past 12 months and a bit is just at its best at that speed. light throttle, 6th gear..
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I’m not sure it ever was. Back in the old days, we used to have three fuel consumption figures - urban, a steady 56mph and a steady 75mph. Of the three, the lowest consumption was usually 56mph.
But given that aerodynamic drag squares as speed doubles, a steady 55 will be better than a steady 56, 54 better than 55 and so on.
In the early days of the Metro, Austin Rover advertised having achieved 83mph from having sat at a constant 30mph - an antique of an engine with a four speed gearbox.
The optimum speed per gear, disregarding drag, is that at which the engine is producing peak torque, hence the smallest throttle opening.
Thus the myth grew about 56mph being the optimal figure for fuel consumption.
Edited by mcb100 on 29/03/2021 at 17:50
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I’m not sure it ever was. Back in the old days, we used to have three fuel consumption figures - urban, a steady 56mph and a steady 75mph. Of the three, the lowest consumption was usually 56mph. But given that aerodynamic drag squares as speed doubles, a steady 55 will be better than a steady 56, 54 better than 55 and so on. In the early days of the Metro, Austin Rover advertised having achieved 83mph from having sat at a constant 30mph - an antique of an engine with a four speed gearbox. Thus the myth grew about 56mph being the optimal figure for fuel consumption.
Exactly right -the general public doesn't understand the difference between specific consumption and actual consumption - specific consumption is at its lowest at the rpm where maximum torque occurs but that road speed varies according to the vehicle's gearing - actual consumption is lower the slower the speed, so 50 has always been better than 55.
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I’m not sure it ever was. Back in the old days, we used to have three fuel consumption figures - urban, a steady 56mph and a steady 75mph. Of the three, the lowest consumption was usually 56mph. But given that aerodynamic drag squares as speed doubles, a steady 55 will be better than a steady 56, 54 better than 55 and so on. In the early days of the Metro, Austin Rover advertised having achieved 83mph from having sat at a constant 30mph - an antique of an engine with a four speed gearbox. Thus the myth grew about 56mph being the optimal figure for fuel consumption.
Exactly right -the general public doesn't understand the difference between specific consumption and actual consumption - specific consumption is at its lowest at the rpm where maximum torque occurs but that road speed varies according to the vehicle's gearing - actual consumption is lower the slower the speed, so 50 has always been better than 55.
Trucks are limited to 56 mph, if you cruise at 52 to 58 you will frequently get in their way and / or will take several seconds to pass them.
Being alongside an HGV should be as short a time as possible (especially for LHD trucks) as you may be in a blind spot, hazardous to your health.
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But you can get better economy figures by tucking in tight behind a truck and getting a good slipstream...
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But you can get better economy figures by tucking in tight behind a truck and getting a good slipstream...
Tailgating trucks? Best of luck with that!
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In the early days of the Metro, Austin Rover advertised having achieved 83mph from having sat at a constant 30mph - an antique of an engine with a four speed gearbox.
Motor Magazine managed over 100mpg with a Daihatsu Charade 1.0 turbo diesel doing a similar speed. It was an actual efficiency test organised, I think, by one of the oil companies.
But as EV's become more common, I can see this type of argument becoming very much back in folks consciousness given the charging time compared to re-fuelling time. In various motoring magazines I read, there are occasionally EV specials, answering questions, debunking myths, etc. But what is often said, if doing a longer journey, is to reduce your speed by 10mph, because if this means one less stop to charge up (which may take more than an hour depending on the car) your overall journey time will be shorter.
One of many advantages Tesla have over the EV competition is not having a lot of styling 'baggage'. As a young brand with no history, they can choose to make the cars as aerodynamic as possible (to maximise the range at highway speeds) as opposed to being tied to what an established brand is expected to look like, which may not be the most efficient shape to slip through the air. I think the Tesla Model S is has the 3rd lowest Cd figure for a production car after the VW XL1 and GM EV1
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I’m not sure it ever was. Back in the old days, we used to have three fuel consumption figures - urban, a steady 56mph and a steady 75mph. Of the three, the lowest consumption was usually 56mph. But given that aerodynamic drag squares as speed doubles, a steady 55 will be better than a steady 56, 54 better than 55 and so on. In the early days of the Metro, Austin Rover advertised having achieved 83mph from having sat at a constant 30mph - an antique of an engine with a four speed gearbox. Thus the myth grew about 56mph being the optimal figure for fuel consumption.
Exactly right -the general public doesn't understand the difference between specific consumption and actual consumption - specific consumption is at its lowest at the rpm where maximum torque occurs but that road speed varies according to the vehicle's gearing - actual consumption is lower the slower the speed, so 50 has always been better than 55.
Trucks are limited to 56 mph, if you cruise at 52 to 58 you will frequently get in their way and / or will take several seconds to pass them.
Being alongside an HGV should be as short a time as possible (especially for LHD trucks) as you may be in a blind spot, hazardous to your health.
If 52-58 is a car driver's chosen speed, then it's best to just match the trucks at 56 and keep pace behind them - the "drafting" behind a big truck will help fuel consumption even at a safe distance, ie without "tailgating"
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If 52-58 is a car driver's chosen speed, then it's best to just match the trucks at 56 and keep pace behind them - the "drafting" behind a big truck will help fuel consumption even at a safe distance, ie without "tailgating"
Unless there has been a huge shift in HGV behaviour, it is impossible to keep pace with lorries without tailgating them.
I always found if I left a safe gap while going at the same speed as the lorry I was following, another one would always overtake into it, and continue at the same speed.
Drop back to a safe distance, and repeat. Just the way it was.
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"Being alongside an HGV should be as short a time as possible (especially for LHD trucks) as you may be in a blind spot, hazardous to your health."
That's exactly what I do now, having almost been zapped by a LHD truck on the M4 about fifteen years ago.The closest I've come to meeting the grim reaper. Now I hang back and wait until there's space ahead, then floor it.
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Optimum mpg will occur at different speeds for different cars. At low power settings ( ie lower speeds) thermal losses become more significant, so eventually they will more than compensate for reduced drag. I vaguely remember a test for the 3 litre Granada in the 1970's and its best figure was 38mpg at 30 mph. No doubt its barn door aerodynamics were a factor. My Mazda 6 2 litre petrol with more power can do better than that at well over twice that speed.
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I vaguely remember a test for the 3 litre Granada in the 1970's and its best figure was 38mpg at 30 mph. No doubt its barn door aerodynamics were a factor.
I remember my dad's 3.0 Ford Zodiac mkIV (same engine) that had aircraft hanger door aerodynamics. You never really got over 20mpg. Good job it had a 19 gal tank!
I think the optimum speed is a combination of various factors inc peak torque curve (revs too low may be less than optimal), aerodynamics and gearing + lots of other things!
I think the optimal with my Skoda Superb 1.4tsi is about 60mph in 6th. At this speed the engine is spinning at about 2000rpm and just nicely really starting to pull well (if you let it!). I'd say it's less than optimal at 30mph in 6th.
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I think the optimal with my Skoda Superb 1.4tsi is about 60mph in 6th. At this speed the engine is spinning at about 2000rpm and just nicely really starting to pull well (if you let it!). I'd say it's less than optimal at 30mph in 6th.
With a 1.4tsi at 30mph you should be in 3rd gear! I can guarantee your mpg will be better at 30mph/3rd gear than 60mph/6th gear. It would also likely be in 2 cyl eco mode most of the time if you have ACT.
False economy to slog a 1.4tsi act engine below 1500rpm.
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I think 55mph was always a general figure, but each vehicle is going to have a different speed it reaches peak efficiency at. In my old td5 I could on a flat road once the engine was warmed up put it in 5th and take my feet off the pedals at 30mph and it would continue under its own power quite happily for AGES until eventually it would start to tail off, usually due to a fractional incline or a corner. Just breathing on the accelerator pedal would solve that. At 70mph it was at 2,500rpm. I know the 4.4 Range Rover diesels sit at 1,500 rpm at 70. I always felt that if I pushed my td5 over 70, it would really start to drink fuel fast. It did come alive between 70-90, but the revs would increase to about 3.5k. Power wise it was dead after that.
It surely depends on the aerodynamics, if you can slither through the air at low rpm you will probably get better mpg at a higher speed, whereas a brick shaped vehicle fighting the wind will probably do better at lower speeds where aerodynamics dont matter quite so much. And gearing, and when the engine is in its torque/power band.
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You need to work out where the sweet spot is for your own vehicle. Where you get the speed the engine is making the a minimal effort ie quietest. But with an acceptable speed. Highest gear cruising. About 63 to 65 in the Vitara.
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My old Skoda Superb with a 1.9 PD TDi 130 engine had a lovely sweet spot. Keep it there while cruising and consumption was in the high 70's mpg. Overall for a longish run would be about 63mpg after some normal driving in town etc. My current Volvo D4 auto is pretty good for a big lump. Cruising at 55mph seems to get great consumption over 50mpg. Not bad for SUV with auto box. It also pays off to know your own vehicle and treat it kindly with a good driving style and good consumption will follow and less wear and tear too.
Cheers Concrete
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Majority of cars display instant fuel consumption values nowadays - either as number or as a bar.
From there it is possible to figure out, in theory, what is the optimum speed for your car.
If it is auto, it should figure it out automatically.
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Majority of cars display instant fuel consumption values nowadays - either as number or as a bar.
From there it is possible to figure out, in theory, what is the optimum speed for your car.
If it is auto, it should figure it out automatically.
I've never found the instant display any use - it varies so much, so quickly even when cruising at steady speed on apparently level roads
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Never really got down to assessing an optimal speed for best mpg, but suffice it to say, that since most major routes became monitored by cameras, my fuel consumption has improved markedly.
Can't think why.
;-)
Edited by Alby Back on 31/03/2021 at 12:43
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