The effect on speed on economy

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Sun 16 Jun 2013 22:24
The effect on speed on economy - Happy Blue!

Just did Manchester - London - Manchester today. In the S-Max TDCi auto with six people on board.

Interesting at what speed the economy starts to suffer by the effects of aerodymanic drag.

I have a rough rule of thumb; the range remaining should always increse for the first half of a tank, if I am driving steadily on the motorway and the previous few hundrd miles was mostly suburban driving. At fill up this morning, the car suggested there was a range of 450 miles. Driving along at about an indicated 72mph, the total of the range remaining plus miles driven since fill up rose from 450 towards 520. At about that point I put some speed on to an indicated 85mph (Nice lightly trafficked motorway). From that point the rate of increase of the combined total reduced and in fact the actual range started reducing, whereas at 72mpg it was always increasing. Reducing the speed down to 72mpg got the total up to 620miles.

One day I drove nice and steadily on a long trip and got a potential 700 miles from a tank (I actually did 520 miles with an indicated 180 remaining).

Shows the effects of drag quite nicely.

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Sun 16 Jun 2013 23:03
The effect on speed on economy - RT

Aerodynamic drag is proportional to the square of the speed - frictional drag is proportional to the speed.

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Mon 17 Jun 2013 00:03
The effect on speed on economy - gordonbennet

Agree with those speeds HB.

Totally different car but our now sold C2 VTS Hdi would happily do 60mpg at 70, but if you upped it to 80 then below 50mpg, not a particularly sleek car though.

I wonder if heavier cars suffer from high speed tyre drag to a similar degree, my lorry when loaded is noticeably more economical at 51mph that it is at 55.

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Mon 17 Jun 2013 00:30
The effect on speed on economy - unthrottled

Why do people use estimated range? It's a meaningless figure. If you want to see the effect of speed on economy (assumng you have a real time log), simply set a constant speed and reset the trip economy figure and allow it to reach steady state over a distance of a few miles. Then change to a different speed and repeat the process.

Power required to overcome air resistance scales to the cube of speed (but the time taken to cover the distance scales with 1/speed), sp, as RT points out, fuel consumption rises with the square of speed once air resistance becomes the dominant force.

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Mon 17 Jun 2013 07:29
The effect on speed on economy - mss1tw

Funnily enough last weekend I set an economy record in my 1.9D Berlingo - 38mpg - 430 miles to a tank vs. 400.

Wait for it...

While towing 350kg of motorbike and trailer.

And loaded up with motorbike gear, clothes, tools.

This is how effective doing 55 - 70 and slipstreaming is.

The next tank got to 444 before the light even came on.

I've got better things to spend cash on than diesel for my work van so this is going to be on-going.

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Mon 17 Jun 2013 08:13
The effect on speed on economy - Happy Blue!

Why do people use estimated range? It's a meaningless figure. If you want to see the effect of speed on economy (assumng you have a real time log), simply set a constant speed and reset the trip economy figure and allow it to reach steady state over a distance of a few miles. Then change to a different speed and repeat the process.

I dont think its meaningless. It may be theoretical, but it can give a more immediate clue as to the economy of the vehicle over a long journey that your method. If I increase speed, there is a fairly instantaneous reduction in the estimate range. If I reduce speed the opposite occurs. The fun is in adjusting the speed in small increments to find which speed produces what effect. Of course it all depends on the sort of driving one has done before the long journey. Several weeks of suburban driving leads to rapid increase in range over the first few miles (at almost any constant speed).

I enjoy these sorts of mind games; of seeking out little bits of trivial information whilst adjusting variables. I do it whilst swimming lengths in the pool to keep boredom at bay.
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Mon 17 Jun 2013 12:54
The effect on speed on economy - industryman

The engine power output requirements for a typical modern family car travelling at a CONSTANT SPEED on a LEVEL ROAD in STILL AIR are as follows:

30mph = 5hp, 50mph = 13hp, 70mph = 25hp, 100mph = 61hp, 150mph = 183hp.

The faster you go, the more fuel you will use, but aerodynamic factors really start to hurt for speeds above 40mph (40mph = 8hp requirement). At 30mph rolling resistance takes 67% of the power, at 70mph 32%, and at 150mph just 9%, (for tyres with a mid range rolling resistance coefficient).

Of course, constant speed/level road/still air figures are not real world motoring. For 30mph constant speed in still air, a 5% gradient trebles the power requirement. Acceleration can use as much power as the engine will produce. A 10mph headwind when travelling at a constant 40mph will add nearly 40% to the level road power requirement.

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Mon 17 Jun 2013 13:37
The effect on speed on economy - RT
Of course, constant speed/level road/still air figures are not real world motoring.

They are at motorway cruising speeds - speed is constant, wind effect reasonably constant unless continually veering and slight inclines/declines cancel each other out.

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Mon 17 Jun 2013 14:13
The effect on speed on economy - unthrottled

t may be theoretical, but it can give a more immediate clue as to the economy of the vehicle over a long journey that your method.

The estimated range is based upon:

1.) the amount of fuel left in the tank-whioch is constantly going down

2.) Driving history since the trip meter was reset

3.) Your instantaneous fuel consumption.

You have three variables and no idea of the relative importance of any of them.The figure is meaningless.

Equally silly is the instantaneous fuel consumption figure. People get all excited when the engine goes into overrun and they're getting 999mpg! Then they get sad when the pull away from the lights and the figure momoentarily goes down to 12.

People should not use data unless they have a grasp of the the underlying principles that give rise to the data. It simply leads to an illusion of understanding.

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Mon 17 Jun 2013 14:24
The effect on speed on economy - Happy Blue!

The estimated range is based upon: 1.) the amount of fuel left in the tank-whioch is constantly going down 2.) Driving history since the trip meter was reset 3.) Your instantaneous fuel consumption.

Quite right and if the range ceases to fall whilst driving the average mpg for that tank is increasing. If the range increases, the economy is improving significantly. I am not trying to make any startling news or quote specific figures for consumption; its just interesting to note the effect of speed on economy.

The figure is meaningless if you regarding it as gospel, but its the change that is interesting, not the figure itself. That is not meaningless. Yes, all three variables have an effect (but after say 50 miles on constant driving, the previous driving history reduces in relevance),

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Mon 17 Jun 2013 14:32
The effect on speed on economy - unthrottled

Quite right and if the range ceases to fall whilst driving the average mpg for that tank is increasing. If the range increases, the economy is improving significantly.

But the estimated range is dominated by how much fuel is left in the tank-which is of no interest unless you're driving across the Nevada desert! If you're interested in the effect that speed has on economy, simply use the instantaneous fuel consumption which takes all the other variables out of the equation.

It's akin to trying to use the tachometer to calculate road speed. You can do it if you have a knowledge of the gear ratios and are good at mental arithmetic-or you could simply get a direct reading from the speedometer.

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Mon 17 Jun 2013 16:12
The effect on speed on economy - Happy Blue!

Yes - I enjoy the challenge!

Besides the instantaneous mpg readout changes to quickly to be useful.

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Mon 17 Jun 2013 17:21
The effect on speed on economy - RT

The estimated range is based upon:


2.) Driving history since the trip meter was reset

3.) Your instantaneous fuel consumption..

Car brands vary in their range algorithm - my Vauxhalls used the average consumption over the last 20 miles - my Hyundai uses the overall average consumption since new

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Mon 17 Jun 2013 18:52
The effect on speed on economy - Bobbin Threadbare

Goodness me I'll have to do all of this in my head as I haven't got a fuel economy gauge any more....

Add some other variables - get the windows open, the aircon on, put some bikes on the roof, allow a large dog with flappy ears to hang out of the window.......

I do recall a discussion from some time ago when there was a large football competition on about those little door flags and the effect they might cause.

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