The 56 mph myth

Author
Discussion

Conian

8,030 posts

200 months

Saturday 13th October 2012
quotequote all
Isnt it because at 56mph air is able to part fast enough to allow objects to pass by reasonably unhindered. Above that speed the car is forcing air out of the way and the drag starts to kick in.

Walk through a busy room of people and people see you coming and step out of the way
Run through the same crowd and you pile into most of them requiring more leg power to keep the same speed

JonnyVTEC

3,001 posts

174 months

Saturday 13th October 2012
quotequote all
No.

Below 56 the air resistance is still less and hence cars are more efficient going slower than 56.

Dave Hedgehog

14,541 posts

203 months

Saturday 13th October 2012
quotequote all
all my cars seam to be the most efficient in there top gear at the lowest rpm where the engines not labouring, speed varies depending on the car

Puddenchucker

4,036 posts

217 months

Saturday 13th October 2012
quotequote all
dcb said:
Puddenchucker said:
dcb said:
...and my car does 30 mpg at 70 mph and 28 mpg at 100 mph.
I'll be amazed if there is any car out there that does only 2mpg less for a 43% increase in speed.
I thought someone would say that.

Velocity isn't acceleration, which is precisely why such figures are
achievable in practice with any number of mainstream cars.

Have a reality check about why the town mpg figure is always less, sometimes
much less, than the out of town mpg figure.

For example, 25 mpg in town, but 35 mpg out on the open road, might
be typical figures.
So you've now changed from comparing two different constant speed mpg figures to comparing urban stop/start to an 'out of town' figure?

I still maintain that no production car will drop only 2mpg when cruising at 100mph compared to 70mph
(assuming it's using the most appropriate gear).

I await to be proven wrong.

Acquah

527 posts

171 months

Saturday 13th October 2012
quotequote all
Puddenchucker said:
So you've now changed from comparing two different constant speed mpg figures to comparing urban stop/start to an 'out of town' figure?

I still maintain that no production car will drop only 2mpg when cruising at 100mph compared to 70mph
(assuming it's using the most appropriate gear).

I await to be proven wrong.
I can think of two scenarios where this could be the case.

One is if you have a vehicle with a frontal area small enough such that increases in speed will not yield significant increases in drag until much higher speeds. If you had a vehicle with a frontal area slightly larger than a human being, then I reckon it's possible. But you're right - no production cars really, and only one or two concept cars. A bike maybe. Or that JCB land speed vehicle thing.

The other is if you have a vehicle which is so inefficient in the first place, that an increase in speed will not affect fuel consumption significantly (I'm not talking about efficiency). Say, a double decker coach. At a 70mph cruise you'll probably get about 6mpg. But if it's a 100mph cruise, that could drop to 4mpg. Yes it's a 33% difference (plausible for the speed difference), but it's 2mpg. Again though, it's not a production car. But I reckon something horrendously thirsty like an old Roller could perhaps come close?

Going back to the other discussion on speed, I think 56mph is picked as an efficient speed because drag increases to the square of speed, so clearly above a certain point it will start getting disproportionately disruptive. ISTR this was around 40-50mph (if memory from my uni days doesn't fail me) so 56mph isn't too far off. This is a simplistic scenario that ignores lots of other factors such as gearing and volumetric efficiency and all that stuff.

That analogy someone posted about the room full of people and making your way across is spot on IMO.

dcb

5,834 posts

264 months

Saturday 13th October 2012
quotequote all
Puddenchucker said:
I still maintain that no production car will drop only 2mpg when cruising at 100mph compared to 70mph
(assuming it's using the most appropriate gear).

I await to be proven wrong.
Really ?

Thousands and thousands of different cars currently on sale and
previously on sale, everything from the four cylinder Nissan Micra to
the mighty BMW 760Li and you're sure of your claim ?

Brave chap.

My previous chariot, an E39 BMW 530, averaged over a mere 85,000 miles
and nine years of ownership, would do 30 mpg at 70 mph, 27 mpg at 90 mph
and 25 mpg cruising at anything up to 154 mph, but mostly 120 ish mph
default autobahn cruise speed.

So there's a real world example of a 3 mpg drop from 70 mph to 90 mph
and a 2 mpg drop from 90 mph to let's say conservatively about 120 mph.

Your claim doesn't look quite so strong now.

Indeed, I checked the numbers from a recent Austrian trip in my
current BMW 325 and got the following

31.3 mpg, max about 100 mph
29.4 mpg, max 155 mph
36 mpg, max 89 mph
28.2 mpg, max 142 mph
28.5 mpg, max 145 mph

Ok row 3 is extreme because I was tailgating Italian lorries,
but the rest of the time, petrol consumption hardly varied from
28.2 mpg to 31.3 mpg. Just look at those maximums, though.
The car got some right treatment.

Cruising at 120 ish mph, with some faster runs up past 140 mph, doesn't
cost a lot of petrol, in a suitable car. I think high gearing counts
for a lot.



otolith

55,899 posts

203 months

Saturday 13th October 2012
quotequote all
Those figures suggest that the engineers at BMW are cretins, if they can barely improve the fuel consumption at 70 over that it achieves when overcoming four times as much drag at 140.

Puddenchucker

4,036 posts

217 months

Sunday 14th October 2012
quotequote all
dcb said:
Really ?

Thousands and thousands of different cars currently on sale and
previously on sale, everything from the four cylinder Nissan Micra to
the mighty BMW 760Li and you're sure of your claim ?

Brave chap.

My previous chariot, an E39 BMW 530, averaged over a mere 85,000 miles
and nine years of ownership, would do 30 mpg at 70 mph, 27 mpg at 90 mph
and 25 mpg cruising at anything up to 154 mph, but mostly 120 ish mph
default autobahn cruise speed.

So there's a real world example of a 3 mpg drop from 70 mph to 90 mph
and a 2 mpg drop from 90 mph to let's say conservatively about 120 mph.

Your claim doesn't look quite so strong now.

Indeed, I checked the numbers from a recent Austrian trip in my
current BMW 325 and got the following

31.3 mpg, max about 100 mph
29.4 mpg, max 155 mph
36 mpg, max 89 mph
28.2 mpg, max 142 mph
28.5 mpg, max 145 mph

Ok row 3 is extreme because I was tailgating Italian lorries,
but the rest of the time, petrol consumption hardly varied from
28.2 mpg to 31.3 mpg. Just look at those maximums, though.
The car got some right treatment.

Cruising at 120 ish mph, with some faster runs up past 140 mph, doesn't
cost a lot of petrol, in a suitable car. I think high gearing counts
for a lot.
So now you're saying you car drops only 2mpg between 100mph & 155mph?

I'll quote the official mpg figures for a 525TDS touring E34:

@56mph = 5.5 litres/100km (51.3mpg)
@75mph = 7.4 litres/100km (38.2mpg)

or put another way a 13mpg drop for a 19mph increase in speed.

Or an Audi 100td 2.5, a car known to be pretty aerodynamic:

@56mph = 4.4 litres/100km (64.2mpg)
@75mph = 6.2 litres/100km (45.6mpg)

that's a 18mpg drop for a 19mph increase in speed.

Or the AA, who state that dring at 80mpg instead of 70 will use upto 25% more fuel?
AA said:
the faster you go the greater the fuel consumption and pollution. Driving at 70mph uses up to 9% more fuel than at 60mph and up to 15% more than at 50mph. Cruising at 80mph can use up to 25% more fuel than at 70mph.
www.theaa.com/motoring_advice/fuels-and-environment/drive-smart.html

ETA

Or What Car, who found a 17% increase in fuel consumption with a Ford Focus diesel when travelling at 80mph compared to 70?

http://www.whatcar.com/car-news/80mph-speed-limit-...

Edited by Puddenchucker on Sunday 14th October 16:06

saaby93

32,038 posts

177 months

Sunday 14th October 2012
quotequote all
Puddenchucker said:
Or the AA, who state that dring at 80mpg instead of 70 will use upto 25% more fuel?
AA said:
the faster you go the greater the fuel consumption and pollution. Driving at 70mph uses up to 9% more fuel than at 60mph and up to 15% more than at 50mph. Cruising at 80mph can use up to 25% more fuel than at 70mph.
www.theaa.com/motoring_advice/fuels-and-environment/drive-smart.html
There's more to it than that. As someone else said you'll get most miles per gallon in the highest gear with the engine at it's most efficient - generally not labouring. But not too high as wind resistance has too much effect.
Some SAABs have a cunning feature where if your fuel gets low it tells you the optimum speed to get to the nearest filling station.
It seems to be keeping in the highest gear, not labouring the engine, gradient up or down etc.
Once it gradually gets you up to speed it seems to be around the magic 50-56mph mark.

Scuffers

20,887 posts

273 months

Sunday 14th October 2012
quotequote all
saaby93 said:
There's more to it than that.
actually. no, not really, aero drag is not linear, once you're over ~50Mph, it;s the most significant factor.

saaby93

32,038 posts

177 months

Sunday 14th October 2012
quotequote all
Scuffers said:
saaby93 said:
There's more to it than that.
actually. no, not really, aero drag is not linear, once you're over ~50Mph, it's the most significant factor.
What I meant was there's more to it than a straight forward 'the faster you go the less economical' described in the link. It applies in the other direction as well. The slower you go the less economcial too.
At 0mph we wouldn't get anywhere so mpg goes to 0 (assuming engine still running).
In between there's an optimum speed of best mechanical efficiency for the given engine and gear train vs lowest aero drag.
They only tested at constant 70mph and 80mph as they were arguing agaisnt 80mph limits
If theyd tried 40mph, 50mph and 60mph on the flat and in an appropriate gear it would give better idea of the shape of the mpg curve
If they had a hilly route other speeds could be beneficial up or down hill


Edited by saaby93 on Sunday 14th October 16:40

coppice

Original Poster:

8,562 posts

143 months

Sunday 14th October 2012
quotequote all
dcb said:
Really ?

Thousands and thousands of different cars currently on sale and
previously on sale, everything from the four cylinder Nissan Micra to
the mighty BMW 760Li and you're sure of your claim ?

Brave chap.

My previous chariot, an E39 BMW 530, averaged over a mere 85,000 miles
and nine years of ownership, would do 30 mpg at 70 mph, 27 mpg at 90 mph
and 25 mpg cruising at anything up to 154 mph, but mostly 120 ish mph
default autobahn cruise speed.

So there's a real world example of a 3 mpg drop from 70 mph to 90 mph
and a 2 mpg drop from 90 mph to let's say conservatively about 120 mph.

Your claim doesn't look quite so strong now.

Indeed, I checked the numbers from a recent Austrian trip in my
current BMW 325 and got the following

31.3 mpg, max about 100 mph
29.4 mpg, max 155 mph
36 mpg, max 89 mph
28.2 mpg, max 142 mph
28.5 mpg, max 145 mph

Ok row 3 is extreme because I was tailgating Italian lorries,
but the rest of the time, petrol consumption hardly varied from
28.2 mpg to 31.3 mpg. Just look at those maximums, though.
The car got some right treatment.

Cruising at 120 ish mph, with some faster runs up past 140 mph, doesn't
cost a lot of petrol, in a suitable car. I think high gearing counts
for a lot.
Sorry but I find it extraordinarily difficult - no ,impossible, to believe that a 1500kg ish big car like a 5 series will do 25mpg at 154mph. But only 30 mpg at 70 mph ..Come on, common sense tells me this just cannot be right!

JonnyVTEC

3,001 posts

174 months

Sunday 14th October 2012
quotequote all
Scuffers said:
actually. no, not really, aero drag is not linear, once you're over ~50Mph, it;s the most significant factor.
What other factors are there?.... It's more like 30 where aero drag is greater than rolling resistance. Probably less cant be bother to do the calls but you can assume a typical tyre has RRc around 0.01.

Scuffers

20,887 posts

273 months

Sunday 14th October 2012
quotequote all
JonnyVTEC said:
Scuffers said:
actually. no, not really, aero drag is not linear, once you're over ~50Mph, it;s the most significant factor.
What other factors are there?.... It's more like 30 where aero drag is greater than rolling resistance. Probably less cant be bother to do the calls but you can assume a typical tyre has RRc around 0.01.
true, but once you are at ~50, it's really all down to aero...

dcb

5,834 posts

264 months

Thursday 18th October 2012
quotequote all
Puddenchucker said:
So now you're saying you car drops only 2mpg between 100mph & 155mph?
No, I'm saying that my old car dropped about 2 mpg between
driving in Germany at anything up to 154 mph, but averaging
maybe 120 mph and driving in the UK, limited to about 90 mph.

Averages aren't maximums.

Puddenchucker said:
I'll quote the official mpg figures for a 525TDS touring E34:
E34 hasn't been on sale for at least 16 years, so somewhat out
of date numbers and it's a diesel anyway, which I have no experience of,
so won't comment on.

Puddenchucker said:
Or the AA, who state that dring at 80mpg instead of 70 will use upto 25% more fuel?
The active part of that sentence is the "up to".

Every person in the world is "up to" ten feet tall,
that doesn't mean we all are over nine feet tall.

Puddenchucker said:
Or What Car, who found a 17% increase in fuel consumption with a Ford Focus diesel when travelling at 80mph compared to 70?
This based on a lowly 1.6 Diesel Focus. Do you seriously think
an engine that small will be in its comfort zone doing
reasonable rpm and so be economical at 80 mph ?

The assertion was made that all cars gave silly mpg at high speeds.

Not true. As BMW and presumably the rest of the
Germans demonstrate, get the gearing high enough and
the fuel consumption at high speed will be very reasonable.

Indeed, I find the newer E91 BMW better than the old E39
and the E91 mpg varies hardly a gnat's whiskers however much I give
it a good sound thrashing at high speed.

But then it's only doing 2700 rpm at 90 mph and so 3600 rpm at 120 mph.
Redline is about IIRC 7000 rpm, so it doesn't work hard to go fast.