Petrol Prices Mean Fuel Efficient Driving

On Wed, 16 Jul 2008 20:28:39 +0100, Mrcheerful wrote (in article ):

Normally on a long journey I get well over 40 mpg in my Toyota Corolla (petrol).

Earlier this year I took the car to Greece on holiday where I found myself on a couple of hundred miles of new motorway. There was virtually no traffic, no service stations, nothing much at all - and a speed limit of 130 kph. I could cruise at a constant 80 - 85 mph for around two hours. I covered a lot of distance but the fuel consumption dropped alarmingly to around 32 - 33 mpg. So that part of the journey cost me an extra £10 or so in fuel and saved me about half an hour in time!

So for me slower is better!

In article , Mike=20 Lane says... ]

Which opens a whole other can of worms. For me, spending another half=20 hour at home is priceless.

--=20 Conor

I only please one person per day. Today is not your day. Tomorrow isn't=20 looking good either. - Scott Adams

On Thu, 17 Jul 2008 00:02:39 +0100, Conor wrote (in article ):

I was on holiday, not trying to get home.

Yes, under inflated tyres do increase fuel consumption. It's also known that inflating tyres to sidewall pressures reduces consumption.

"Mark W" gurgled happily, sounding much like they were saying:

By "sidewall pressures", do you mean the maximum safe pressure moulded onto the sidewall?

Yes, it may reduce rolling resistance and therefore fuel consumption slightly, but it'll also drastically reduce grip and increase tyre wear.

Yes. But not _56_ mph for _all_ cars. that's the fallacy...

Yep, complete twaddle. In general it's at about the slowest speed that you can maintain it top gear, more or less. Wind resistance builds to the cube of the speed, above about 40mph wind resistance is the dominant factor.

I remember quite clearly the advertising for the Mini Metro when it was first launched '' 80mpg'' was the claim, the small print said ''at 30mph''!

Julian.

And how much per hour were you paying for your holiday?

I knew someone that could coax 60mpg on a run from an early metro, he must have driven so gently!!. I have had several metros of all the available engine types (petrol and diesel) and the norm was 40mpg or a little under.

You must have got more than 40 from the Diesel version, ISTR it was that rather troublesome all alloy unit that went into the 106 etc, my mate gets almost 70mpg from his. Back in the days when I had a 205D I had 59mpg very easily and that was a 1.7L. So much for progress when cars like the 1.8 Marina easily got a better MPG than almost all comparable cars of today.

I remember the advertising hoardings advertising 80mpg from the Metro, god knows how they got it, but I assume they did?

Julian.

On Thu, 17 Jul 2008 17:29:25 +0100, PCPaul wrote (in article ):

I understand the point you are making but I don't think it's really a very meaningful calculation. More to the point I would have thought is what does one actually do in the time that one saves? In my case I'm afraid the answer was "not a lot"

it was a 1.4 and local use gave only about 40, I never checked it on long runs, it wasn't that sort of car, but it might have done a few more. I understand that the 1.5d was much better

And even under that sometimes. ;-)

Years ago I designed, built and raced an electric vehicle (mine happened to be in the form of a motorbike) for the endurance type 'racing'.

The naked bike with me quite prone would pull an average of 30A, at

25mph, in a particular gear [1]. I then made and fitted a nose fairing (for the bike not for me ) and with everything else the same, and in spite of the extra weight of the fairing, the speed went up to 30 mph and the current down to 25A.

I guess with an aeroplane you want to go fast enough to balance lift with efficiency and the final (air) speed is the just a function of schedules etc. Having to lug spare fuel about is even worse for them of course (well, not doing it is worse for them).

There are also other factors, like rolling resistance. This old Rover

218SD seems to be very 'free rolling' and needs the handbrake on even the tinyest of gradients. Freewheel experiments down hills often mean I can hit pretty high speeds very quickly and on the hills I've tried so far that acceleration doesn't seem to slow down much with speed!

All the best ..

T i m

[1] Averaged over an upwind and downwind section of the track. MIRA was good place for that, it was the loop ended track at the bottom of this pic:

Wind resistance, or aerodynamic drag, increases with the square of speed not the cube. Power required to overcome aero drag does increase with the cube of speed though. The equations behind this can be found here.

Thanks, I like this table from your site, it makes a nonsense out of such claims as ''my car has a sweet spot at 70-80mph, drive slower and fuel consumption worsens''

SPEED (MPH) FLYWHEEL POWER REQUIRED MPG 30 5 97 40 8 76 50 12 60 19 48 70 27 38 80 38 31 90 52 26 100 69 22 110 90 18 120 114 16 130 143 14 140 176 12 150 215 10 160 258 9 170 307 8

On Sat, 19 Jul 2008 10:58:06 +0100, Julian wrote (in article ):

The table certainly agrees almost exactly with my experience in practice.

The speed/mpg curve very much depends on vehicle weight. Heavy vehicles are less sensitive to reductions in speed because they have high rolling resistance drag and therefore poor fuel consumption at low speed even when there's almost zero aero drag. If I change just the vehicle weight in that table from 2500 lbs (A) to 4000 lbs (B) the mpg then becomes..

Speed...(A)......(B)

30........96.7....69.4 60........47.7....39.9 100......21.7....19.9

Not much difference at 100 mph but a 30% drop in mpg at 30 mph. Combine that effect with the increasing inefficiency of the engine at low power outputs and big heavy cars with large engines can end up being almost totally insensitive to speed - similar mpg from 30 mph all the way to 80 mph.

Light vehicles with low rolling resistance benefit much more from driving slower as the power required to overcome aero drag drops. During the fuel crisis in 2000 I did a 188 mile trip in my 2000 lb Fiesta XR2i at 50 mph on the motorways and 40 mph on other roads. It was hellishly boring but there were no other vehicles on the road for me to hold up so I persevered. From a car that normally averaged 34 mpg I got 51.5 mpg on that trip.

Conversely my current Focus which weighs about 3000 lbs with me in it and has much wider tyres hardly benefits from doing 60 mph on the motorway rather than 80 mph. I can get between 36 and 38 mpg driving normally on the motorway but can't get it to exceed 40 mpg no matter how I drive. If it weighed the same as the Fiesta I suspect things would be very different.

As a general rule a petrol engine will only be operating reasonably efficiently if it's being asked to produce at least 15% of its rated power output. As you drop below that the cylinders aren't filling with enough air to burn properly and internal engine friction becomes a large part of what the fuel is being burned to overcome. Even at idle when an engine is doing no useful work it burns a surprising amount of fuel just to overcome internal friction. At 30 mph an average car only requires 5 bhp to propel it. Asking a 130 bhp engine to produce just 5 bhp is not an efficient proposition so real world fuel consumption at low speed will not be as high as my table suggests although if the engine was small enough it could be. A

500cc 30 bhp engine could operate quite nicely at that speed and probably really would show nearly 100 mpg. A 2 litre 130 bhp engine would struggle to get 2/3 of that.

So the answer to good fuel economy is very simple. Drive small, light, aerodynamic cars with small engines and drive them slowly.

The other anomaly wrt modern petrol engines is that they always run a stoichiometric mixture at part throttle, to allow the three-way cat to operate most effectively. Older vehicles would run a thin mixture at part throttle and gain about a10-15% fuel consumption advantage . With fuel prices going ballistic isn't there a good argument to ditch all this silly emission legislation and accept a tiny bit more CO and oxides of nitrogen in return for greater vehicle fuel economy?

Julian.

My own opinion is yes. It's very hard to beat the economy of a well setup SU carb and nothing else comes close as regards cost and ease of manufacture. We are currently forced to pay hundreds of pounds per car for FI, cats, lambda sensors and all the other crap required to minimise emissions when an SU can beat the economy of modern vehicles for £20. I shudder to think what the actual carbon footprint is of the manufacturing costs of all the modern high tech emissions crap but I'm damn sure that overall we've been going backwards not forwards for 20 years.