Driving Economically - 56mph Myth??

jdm01

shown73 wrote: »

I'd be interested if someone with one of those whizz-bang fuel computer thingies could run a test on slipstreaming. I used to have a mk3 Zodiac, manual, (remember them?), and travelled about 30 miles on the then new M40 to Oxford, to work. Well, this thing did 18mpg, whether driven briskly or sedately, and I spent ages behind lorries trying to improve this, but it didn't make a blind bit of difference. It's bothered me ever since, because in theory it should have. Maybe we just weren't travelling fast enough to make a difference, I don't know, but with all the modern widgets, if anyone is seriously bored one day, they could have a go for a few miles, just to see if it works. I wouldn't want to spend a long journey like that, but just as a matter of interest......

I know it's been a while, but did anyone ever try this? (not that i'm offering)

J

goldspanners

i find it hilarious watching someone do this in the rain,with all the spray around, wipers on full pelt,right up the !!!!! of an artic. crazy!

thescouselander

shown73 wrote: »

I'd be interested if someone with one of those whizz-bang fuel computer thingies could run a test on slipstreaming. I used to have a mk3 Zodiac, manual, (remember them?), and travelled about 30 miles on the then new M40 to Oxford, to work. Well, this thing did 18mpg, whether driven briskly or sedately, and I spent ages behind lorries trying to improve this, but it didn't make a blind bit of difference. It's bothered me ever since, because in theory it should have. Maybe we just weren't travelling fast enough to make a difference, I don't know, but with all the modern widgets, if anyone is seriously bored one day, they could have a go for a few miles, just to see if it works. I wouldn't want to spend a long journey like that, but just as a matter of interest......

Could be interesting. I have a instantaneous MPG needle on my car so it should be immediately obvious how well it works. I have tried this while cycling a few times, works great behind busses but you need to get close - a bit to dangerous really. I'll try it in the car though next time I go on the motorway (leaving a reasonable braking distance).

vikingaero

Slipstreaming does work. But to get the full effects you need to be about a foot from the car in front. The driver of the car in front won't be happy and Plod will pull you in an instant.

thescouselander

harveybobbles wrote: »

Think it depends on the car's engine/gearbox combination.

I had an Audi A4 1.8T which was more economical at 65 than it was at 55...
I replaced it with a BMW 528Auto which was at its best doing 84mph.

It's all to do with gear ratios etc...

I remember doing some analysis on this during my degree. I cant remember exactly how it worked out but about 55 mph was the best speed to cruise at due to the relationship between drag and distance covered per unit of fuel - this was independent of engine and gearbox efficiencies. Maybe I'll try and dig out the notes next time I'm in the loft.

jackogoesglobal

Contrary to previous responses and answers all over the web this is not an arbitrary figure. It corresponds to the speed at which the air flow around an object changes from laminar flow to turbulent flow. It always occurs at this speed, independent of the shape and size of the object and is therefore the same for all vehicles travelling through air. At this speed there is a sudden drop in the air resistance, a blip in the graph where the general trend is increasing resistance, or drag, with increasing speed. Assuming the only other resistance is from the tyres and that this is similar for all cars then the major force that the car has to overcome is the air resistance. Hence 56mph is the most fuel efficient speed ! No wonder then that car manufacturers publish data for their cars at 56mph as it will show them off at their best ! Also it is a happy coincidence that 56mph = 90kmh and therefore the speed limit on the continent. Presumably the limit in the uk was rounded up to 60mph but it could just have easily been 55 . . .

redux

jackogoesglobal wrote: »

It corresponds to the speed at which the air flow around an object changes from laminar flow to turbulent flow. It always occurs at this speed, independent of the shape and size of the object and is therefore the same for all vehicles travelling through air. At this speed there is a sudden drop in the air resistance, a blip in the graph where the general trend is increasing resistance, or drag, with increasing speed.

Absolute nonsense I'm afraid

Of course airflow depends on the shape of the object, and some is laminar at much faster speeds than that.

AdrianHi

movilogo wrote: »

And remember, if your tyre pressure is not correct, you'll burn 10-20% more fuel irrespective of how good you drive or not.

According to RAC figures
If your pressure is 20% down it increases fuel consumption by 3%.
It's also dangerous, wearing the tyre around 20% faster and you run the risk of a blowout at motorway speeds. 6% of fatal motorway accidents caused by under inflated tyres. 75%-85% (depending on whoes figures your reading) of blow outs cause dby under inflated tyres.

tomstickland

jackogoesglobal wrote: »

Contrary to previous responses and answers all over the web this is not an arbitrary figure. It corresponds to the speed at which the air flow around an object changes from laminar flow to turbulent flow. It always occurs at this speed, independent of the shape and size of the object and is therefore the same for all vehicles travelling through air. At this speed there is a sudden drop in the air resistance, a blip in the graph where the general trend is increasing resistance, or drag, with increasing speed. Assuming the only other resistance is from the tyres and that this is similar for all cars then the major force that the car has to overcome is the air resistance. Hence 56mph is the most fuel efficient speed ! No wonder then that car manufacturers publish data for their cars at 56mph as it will show them off at their best ! Also it is a happy coincidence that 56mph = 90kmh and therefore the speed limit on the continent. Presumably the limit in the uk was rounded up to 60mph but it could just have easily been 55 . . .

Do you have any sources for the claim that it is the speed at which flow becomes turbulent?

redux

tomstickland wrote: »

Do you have any sources for the claim that it is the speed at which flow becomes turbulent?

If the answer contains Reynolds numbers, there's a chance this person knows what they are talking about. Otherwise not