Graph of car fuel consumption versus speed

[The Natural Philosopher]

I think so too.

Certainly maximum welly at ultra low RPM is not ideal.

[The Natural Philosopher]

Ah, Thats for over 7.5 tonnes tho, and Im not licensed for them, so I never learnt that ;-)

Mind you, none of them keep to it. I drive the A11 frequently in a 3 ton camper, and I keep pace with the artics.;-)

If they slow to 40mph, it creates a tailback from Thetford to the start of the dualled section..all ten miles of it :-)

[Ed Sirett]

Have to diasagree the Reynolds numbers for object of size car, in air, mean that even a few mph and you are into unambigously turbulent flow, where the drag is proportional to speed squared.

The Cd can be affected by things like a half closed door/hatch. But is largely unaffected by speed for Reynolds numbers of 10K+.

[Ed Sirett]

IIRC about 20 years ago, 30 mph, was about evens between the air drag and other resistances (mainly tyres). These days the shapes are more slippery so 40mph might be the balance point.

It all depends on what you mean by 'significant' and 'dominant' If 40 mph was the balance between aero drag and other friction then At 60mph aero drag is about 70% of total which I call significant.

[Harry Bloomfield]

It happens that Ed Sirett formulated :

I've done a bit of caravan towing rough research using the fuelling computer and given still conditions and fast roads, my most economical speed is around the 50 to 55mph mark - surprisingly, because you would think the poor aerodynamics would make it very uneconomical. When in no particular hurry, I can manage economy figures only slightly worse than when not towing.

[Andy Champ]

Bol... err.. that turns out not to be the case. Keep the revs near the torque peak, and the throttle butterfly wide open so it isn't b***ing up the airflow and wasting lots of power. Remember of course that a lot of engines will enrich the mixture at full throttle to get max. power, and so you want to avoid actually flooring it. Diesels BTW with no butterfly are far happier at lower "throttle" openings.

Usually you can spot the prat, and stay well clear. I've never actually had to resort to the hard shoulder.

Andy

[terry]

Hmm!

10 cubed =3D 1000 20 cubed =3D 8000 30 cubed =3D 27000 40 cubed =3D 64000 50 cubed =3D 125000* 60 cubed =3D 216000* Maybe the fact that it almost doubles between 50 and 60. And: 63 cubed =3D 250047 (Hey that's double the number at 50 so anyone going 'just a bit over' not plus ten per cent or anything! Just a thought anyway. PS. Seem to remember from wind loading on radio antenna on tall towers; something about Pressure =3D 0.003 times the wind velocity squared???? For example at 30 mph; p =3D 0.003 x 900 =3D 2.7 lbs per square foot. (Roughly say 3) Whereas at 100 mph p =3D 0.003 x 10,000 =3D 30 lbs per sq.ft. So that's at variance with the cubed theory?

[The Natural Philosopher]

That is consistent with my understanding as well.

Airflow behind a car is fully stalled from a few mph onwards.

>

[The Natural Philosopher]

No, pressure or force is not power. The pressure is squared, the power is cubed

[Roger Mills]

You're obviously a quick learner!

[The Natural Philosopher]

sure. Once reminded it all comes back. :-)