Economic driving

In message , Douglas Hall

Actually using the a/c uses less fuel than running with the windows down.

Tim..

Manual.

Using the instant readout and driving at different speeds, it shows that between 2500-3000rpm mpg is lower than when over 3000rpm. This is in 5th gear.

I have not experimented with 4th gear at the same speed to keep the revs to

I am reasoning is that torque output below 3k is low and the engine is effectivly labouring with a wider throttle opening than that at a higher roadspeed.

I would add its the 2.5 10v engine which does about 27mph / 1000rpm in 5th. (100mph indicated= 3600rpm)

Tim..

PS this is with the cruise control on maintaining a constant speed.

Tim..

Tiny bit of a generalisation there Tim. At some speeds in certain cars with the windows down to a certain level, I'll concede you may be right, depending on the level of cooling required, etc...

Mine is an auto - and I find the opposite, really. 70 indicated is around

I get much better averages, according to the computer at least, by sticking under 80, and better the closer I am to 70. Haven't experimented for below

Same engine for me.

I can't remember on the speeds per 1k revs in top, but 70 is around 2600rpm,

On the motorway, though, at normal motorway speeds, it's more likely that the window being down rather than using the aircon is likely to introduce more drag aerodynamically, than the compressor on the engine - unless the engine is particularly weedy.

But why would anyone need aircon at motorway speeds? Why not just use the internal vents?

OK, I can see this may not be sufficient in some black cars on a really hot day, but anyone stupid enough to buy a car that colour can stew, IMO.

Even worse are those colours that are almost black, but not quite. It may be some kind of metallic black. Hence very difficult to repair scratches.

Simple, when the temperature of the air coming through the vents is still warmer than you want.

On some of the warmer days, I'll be using the A/C whilst I'm on the motorway. Typically, I find, when the outside temperature is over 23C I'll be using it a bit. 25C and above, maybe quite a lot. 28C and probably constantly.

Now I'll buy that the 25C+ days are dwindling, now, but most days when driving home it's low twenties, and I'll use it a bit.

I said 'need'.

Well, no-one needs to drive a car, or even live, where do you want to stop?

Christian.

In which case, there's likely no true _need_ for cooling air through the vents, either. Nor to be able to listen to the radio and / or music. And clearly, the best fuel economy is likely to be achieved with as little electronic or mechanical accessories being used - either in terms of alternator drag, mechanical drag through auxiliary drive belts, or aerodynamic drag.

However, the context of the discussion was regarding cooling - and likely whilst at motorway speeds. And if somebody _wants_ the interior to be cooler than it is (perhaps due to ambient, perhaps due to solar effects) then they have numerous choices. Airflow through the vents - either by merit of road speed, or by the fans, may not cut it. And at motorway speeds, aircon is probably going to be more favourable compared with opening a window, in terms of fuel economy.

But as you rightly imply, drivers most likely won't die or be injured by such heat. The may likely just be a bit hot under the collar, or sweat a lot - kudos for stressing the importance of your choice in words in using "need".

However, if somebody wasn't sure which option would be preferable in terms of fuel economy - and given the context of the OP post, that seems a reasonable proposition - I see nothing wrong in informing them.

Well the subject is 'economic driving' so I assumed the OP wanted the best mpg he could reasonably hope for. Driving without aircon in the UK is hardly a big deal, IMO, but then I'm not some soft southern pooftah who can't function outside of a micro-controlled environment.

Have you been looking in my garage?

I'm may be some soft southern pooftah, but I don't even have aircon! When I had a car that did, I did use it quite a lot, though, even when it wasn't that hot. The dehumidifying was very comfortable.

Christian.

I've certainly seen more cars doing dead on 70, and not the ones you'd normally see, Audi, VW etc.

The White Van Men are still driving like total muppets as usual.

I'm doing the same this week. Partly due to uncertainty about fuel, and partly to see what I can do.

Over 2 days, I've now averaged >60mph, which I'm pretty impressed at. THis is where I find cruise handy. It's a little lighter with its right foot than I am!

Pete.

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Colin Stamp saying something like:

There's a secondary region of calm air behind a truck, Some 40 t0 60 feet behind it, the airflow has curled round the back of the truck and calmed down again, seeming to leave a slightly low pressure area. This works for box-bodied rigids and most trailer units travelling at 60mph.

Try it on a bike; you really feel it. I first discovered this when I was on the motorways during my first biking winter on a slow old shitheap and didn't have a decent fairing or proper bike gear. That calm air region made a 400mile journey in February bearable.

The point I'm making though, is that same calm(er) air region can help you slipstream without incurring the interest of the dibbles.