Driving Economically - 56mph Myth??

big_gay_kirk

All I xcan say is that AA tests in the 70s showed that a 2 litre Cortina mk 3 did more mpg at 30 than at forty, and that consumption increased as speed increased.. IIRC it jumped horrendously after 60mph... and at seventy it was really bad... I'll try to find the figures....

big_gay_kirk

here we are... from the AA book of money saving motoring.... in 4th gear, here are the consumption figures at constant speed for a Ford Cortina 2000 (thats a Mk 3 by the way....)
10 mph.... couldn't go.. but did 26.3 in 3rd
20mph... 37.7 miles
30mph.. 37.7 miles
40mph... 37 miles
50mph 33.7 miles
60mph... 29.8 miles
70mph... 25.7 miles
80mph... 21 miles
90mph.. 16 miles
in separate tests, they found that traffic conditions affected it thus..
rush hour 17.5 mpg
suburbs 22mpg
less congested 29 miles
winding country roads.. 31mpg
a roads 34mpg
motorway 24.5 mpg..

and driving technique..
n"normal" 26.5mpg
very quiet 32.7 mpg
hard 20mpg

I know its old data, but it just goes to show.. back then 50mph was quoted as the usual "economy" speed.. but it weren't so back then!
(source AA:Money-saving Motoring, London 1974)

jackogoesglobal

Yes, this is to do with Reynolds Number. As for 'do I know what I'm talking about', well, I was taught this during my mechanical engineering degree most of which is a distant fog. I'd have to dig out my notes to show you the equation and proof and remind myself the exact details. I'm sure this will be explained in any decent Fluid Mechanics course book. However, what has always stuck in my mind is this strange fact that the change in flow from laminar (smooth) to turbulent flow occurs at the same airspeed and is completely independent of the size and shape of the object (ie car). I'll dig out my notes . . .

bryanb

jackogoesglobal wrote: »

Yes, this is to do with Reynolds Number. As for 'do I know what I'm talking about', well, I was taught this during my mechanical engineering degree most of which is a distant fog. I'd have to dig out my notes to show you the equation and proof and remind myself the exact details. I'm sure this will be explained in any decent Fluid Mechanics course book. However, what has always stuck in my mind is this strange fact that the change in flow from laminar (smooth) to turbulent flow occurs at the same airspeed and is completely independent of the size and shape of the object (ie car). I'll dig out my notes . . .

Been patiently waiting for your notes, Very interested to know how the airspeed is arrived at.

tomstickland

I'll do some research into it, but I belive that at all meaningful road speeds the air flow will be turbulent.

bryanb

tomstickland wrote: »

I'll do some research into it, but I belive that at all meaningful road speeds the air flow will be turbulent.

If that's so, what is the point of streamlining? Is it perhaps less turbulent if streamlined?

tomstickland

I've got the results from my 1700 mile trip to France and back.
This is based on driving at a near steady speed on motorways, though I did slow on large hills to keep the indicated instantaneous mpg above 30mpg

Mway at 70mph, plus 10 minutes at around that plus 30: 39mpg
Mway at 70mph, being very careful: 42mpg
Cross country and around half a tank on motorway at 70: 39mpg
Mway at 85mph indicated, slowing on big hills: 37mpgs

I'm just finishing off the last tank, which included a long session driving with little regard for fuel economy, I'll update on that soon.

I'm very impressed with these results - this is in a 1990 BMW 318iS (1.8 16v) with 220,000 miles on the clock.

There's about a 5mpg improvement in reducing the speed from 85 to 70mph, which is a 13% improvement for driving around 13% slower.

Just for fun, I worked out that at 70mph it's costing 15p per minute and at 85mph it's 21p/min. Obviouslly at the faster speed less minutes are spent, but as I was sat on the road for 14 hours yesterday, I did wonder what the cost per minute was.

AdrianHi

tomstickland wrote: »

I've got the results from my 1700 mile trip to France and back.
This is based on driving at a near steady speed on motorways, though I did slow on large hills to keep the indicated instantaneous mpg above 30mpg

Mway at 70mph, plus 10 minutes at around that plus 30: 39mpg
Mway at 70mph, being very careful: 42mpg
Cross country and around half a tank on motorway at 70: 39mpg
Mway at 85mph indicated, slowing on big hills: 37mpgs

I'm just finishing off the last tank, which included a long session driving with little regard for fuel economy, I'll update on that soon.

I'm very impressed with these results - this is in a 1990 BMW 318iS (1.8 16v) with 220,000 miles on the clock.

There's about a 5mpg improvement in reducing the speed from 85 to 70mph, which is a 13% improvement for driving around 13% slower.

Just for fun, I worked out that at 70mph it's costing 15p per minute and at 85mph it's 21p/min. Obviouslly at the faster speed less minutes are spent, but as I was sat on the road for 14 hours yesterday, I did wonder what the cost per minute was.

Want to throw your figures into the pot on a similar discussion going on, on a BMW enthusiasts forum right now?
One of the themes on this particular discussion is the effect of increasing speed on economy and how it affects different engine and gearbox combinations differently.
Big engines see a much slower increase in fuel used as speed increases.

tomstickland

Well, yes, fuel consumtion is proportional to;
1) power requirement
multiplied by
2) bsfc (brake specific fuel consumption) at that speed/load combination.

So, you would expect an increase in speed to cause an increase in fuel consumption, but the rate of increase would depend on the engine bsfc performance. Small engines have to be thrashed to put out high power, whereas large engines don't. It's all about running the engine near its optimum power output.

So a 1.1 Petrol engine might be most efficient making 30BHP, so a steady 65mph cruise is its optimum bsfc point and the power requirement is quite low, so overall mpg is good.

A 3.0 engine might be at optimum bsfc at 80BHP, which would require a speed of about 90mph. So it would be running at its best bsfc, but the power requirement would be higher, hence worse mpg overall.

loofer

There's no science to back this up but here's my theory anyway.

I think most efficienct for 'my car' is in top gear (6th) where the torque band is at it's peak.
For my VW Bora TDI this starts from about 1950rpm I think.

Thankfully for me - 2000rpm is 70mph...so observing that speed limit is most efficient contsant speed - me thinks.

Diesel car's have alot more torque than their petrol equivalants and this peak torque is achieved much lower down the rev range.