O/T (Sorry) But I was wondering Is this true?

So a vehicle at 80 is quieter and uses less fuel for a given measurement of distance than one at 60?

I want to go on this fuel economy course you've been on. Rather than do the serious stuff like conversation of energy, managing the deceleration, hazard anticipation, acceleration sense and gear selection, no instead they show you how to bend the laws of physics.

Damn and I thought I was doing something useful with my time.

Winding anybody up isn't good cricket. Labouring your engine at 1,500 rpm in 5th isn't good news.

Most idle around 850RPM. It takes very little throttle to raise this to

1500RPM meaning that it'd be a bastard to maintain 30MPH@1500RPM because you'd have to be very light on the throttle.

And correct. The charts I've seen showing wind resistance vs speed have a fairly flat line that increases exponentially when you get above

53MPH.

From Daves post:

I'm in 5th by 30mph on the flat and certainly by 40mph up steady inclines (and this is the 5th car I've owned with 5 gears over a 20 year period and all the same basically).

The key is to have a small enough engine that you don't have to be.

These newfangled lacking-in-torque 16 valve lumps don't help much either. My SO 8v 2.0 Cavalier would quite happily (and economically) run round town at under 2000 revs and pull away fairly smartly too.

I think if you take a Conor 'wrong' as meaning 'not necessarily, there are certain cases where that may not hold' you'll be closer.

I can certainly make my car drink more and be noisier at 60 than 80. Even at

1. And have a damn sight more fun with it too ;-)

S'funny that. You'd think me being a specialist in fluid mechanics would have heard about it by now if the laws of physics changed at 53 mph. Drag is proportional to the square of speed at any speed. The only reason the lower speed part of such a graph looks flat compared to the higher speed ranges is because of the scale of the Y axis. If you inflated the scale to show just the 0-30 mph range it would look exactly the same shape and the same if you deflated the scale to show 0-300 mph or any other range.

Aww Thanks. There goes my thesis :-(

There won't be one single definitive curve because it varies so much with vehicle size/shape and engine size/efficiency. As a general rule small cars with small engines get significantly better economy the slower they go and large cars with large engines have a flatter consumption/speed relationship that optimises at higher speeds. On the following page on my website..

I look into the equations that link speed, drag, power and fuel economy. However even if you know how to calculate the flywheel power required to propel a vehicle at a given speed (which is very simple) you have to factor in the engine economy at different rpm and throttle openings and also the internal frictional losses in the engine which have to be overcome before any useable flywheel power is produced. These are not simple matters and will be vehicle specific.

The table I show on that page which gives the theoretical fuel consumption at different speeds is clearly not correct at very low speed. The reason is that engines don't operate efficiently at low rpm and power outputs and also the internal frictional losses become a significant part of the required net flywheel power output at very low throttle openings. Even if you only require a theoretical net 5 bhp to maintain a steady 30 mph a 2 litre car engine is probably having to burn enough fuel for nearly twice that to overcome its own internal friction.

However, if you make the engine small enough, say a single cylinder scooter engine that only produces a maximum of 10 bhp anyway, the frictional losses become so small and waste so little fuel that the theoretical consumption figures become possible.

Once an engine gets into its efficient working range which will be between about 20% and 70% of its peak power output the chart I show becomes fairly accurate. For an average engine of say 100 to 130 bhp that efficient range will correspond to speeds between 60 and 110 mph. For an average car it shows a consumption of 38mpg at 70 mph which is clearly not unreasonable. My old Fester XR2i used to achieve just about exactly that. However I'm quite sure it wouldn't have got 97mpg at 30 mph although a small motorbike or scooter might well do so.

The main point ought to be clear though. If we drove lighter cars with smaller engines our economy would be somewhat better at any speed and much better at low speeds.

Unfortunately cars become heavier year by year because of things like crash protection structures, airbags, sound proofing and electric everything. A

1980's hatchback weighed about 900kg. A 2000 hatchback like my Focus weighs 1250 kg. That extra mass needs to be dragged about everywhere just to move a 70kg human and hurts economy in the process, especially at low speeds.

my granada is more economical at 95 mph than it is at 75 mph :)

'less uneconomical', shurely?

dojj ( snipped-for-privacy@dojj1.fsnet.co.uk) gurgled happily, sounding much like they were saying :

My grandad cost about the same at 95 as 75. The care home fees about broke even with his reduced whisky consumption.

One thing to consider with consumption is the cost of braking. I'd wager that the cost of going 0 to 60 to 0 back to 60 would be more than

0 to 50 to 0 back to 50. One could argue that the speed bumps introduced in urban areas increase emissions as most people tend to slow for the bump then speed up then slow for the next etc. The same applies to sections of road with a high traffic volume - higher speeds usually mean more variance in the actual speed.

As for the 30mph in 5th thing many low powered cars are geared very high so that they can accelerate reasonably. I'd get very annoyed if my car could happily do that. In fact I'm a little annoyed that one of my cars is happy at 30 in 4th. If I *was* doing 30 in 5th even full throttle wouldn't have much effect on the speed or revs of the car.

( snipped-for-privacy@yahoo.co.uk) gurgled happily, sounding much like they were saying :

Until such time as 50 isn't less than 60, then I'd hazard a guess that you might be right...

There is no argument, it does.

Ya, but hey, this is Usenet...

Holding on to 1st at 30 isn't fun though, is it?

Hmm. I'd like to prove, or otherwise, that one. :)