Mon
Feb 08 2010

What comes up must go down

Tim Dickson
Being a frugal man of simple means I think I spend more time with an eye on our test cars’ fuel consumption than some of my more leaden-footed colleagues. That and having a 36-mile each-way commute. Actually, the journey is quite an easy one. I’m on the M40 motorway before I’m in third gear of a morning, then it’s the M25. a stretch of the M3 and, finally, a couple of miles of urban crawl before arriving at the office.

I’ve often noticed that I’m able to record noticeably better fuel consumption on the way to work in the morning compared with the evening drive home. To be honest, I’d not given it that much thought. The morning traffic tends to flow more smoothly than that in the evening, so I suppose I’d absently mindedly put it down to that.



Peugeot 5008 road test

However, I recently spent the weekend with Peugeot’s 5008. It featured, among many other things, a readout giving the car’s current altitude above sea level on the LED display. I’m sure this is something that appears on many other cars’ sat-nav/infotainment screens, but for some reason it really caught my eye in the Peugeot.

And so it was one morning that I noticed my house sits at an altitude of 522 feet above sea level (or 159 metres if you’re in France), nestled as it is on a rocky outcrop in the Chilterns. And 36 miles later I again noticed that Autocar’s riverside offices are just 59 feet (18 metres) above sea level.

No wonder fuel consumption is better on my journey to work compared with my journey home; on the return trip not only does the car’s engine have to propel it 36 miles along the horizontal, it also has to winch its one and a half ton mass (or thereabouts) 463 feet up in the air.

Perhaps there’s something in the notion of the ‘two-way average’ after all.


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Comments

MrJOD February 8, 2010 4:42 PM

There may be something in it, but I suspect that it's the smoothness of traffic flow which makes the most difference.

My commute, in an E46 330i, is between two locations no more than 30ft different in height (BMW satnav displays height as well), but depending on the traffic I can see anything from 23mpg to 31mpg on the same run.

Imagine the CO2 reduction if the govt actually configured junctions and traffic lights to optimize smooth traffic flow rather than, in many cases, deliberately disrupting it.

I reckon it might result in at least a 10% reduction in CO2 output, not to mention reduced frustration and shorter journey times.

beachland2 February 8, 2010 4:53 PM

The pioneer spacecraft went up and they will never come down, they have left the solar system. Time to kill that old expression.

fredwest February 8, 2010 5:17 PM

You do know the prevailing wind in UK is generally a westerly?

Most of your journey into work is generally eastwards, so a tailwind; your journey homeward generally westwards, so a headwind. If you've ever been on a pushbike, trying to maintain a decent pace, say 20mph, you'll know the effort difference required between a heawind ride and a tailwind ride. Just because you are cocooned inside a metal box doesn't make this physical force - to part air - go away. Driving an MPV, with a drag coefficient around 0.35-0.40, exacerbates the problem further.

The rest of journey, which isn't in a generally east-west axis, is the M25, on a north-south axis. Again, in recent times in UK - cold winter weather - the general airflow has been around a northerly direction, when it has not been the more typical temperate, west/south-west direction. This explains further the wind assist in the morning and the wind resist in the evening return journey.

If in any doubt as to the difference a prevailing windflow can make to journey times/fuel economy at a given speed, bear in mind transatlantic flights in the jet stream, where a west to east flight can take one or two hours less.

So, in conclusion, I'm tempted to say your explanation, as to fuel economy difference, between to-work/return-from work journies, is nought but a tall story.

disco.stu February 8, 2010 5:55 PM

Also bear in mind that if you haven't had breakfast but you do have decent lunch, you will be heavier on the way home than on the way to work. That should also make some sort of difference. Unless you are Michael Schumacher, who was reported on the Autocar forums to be a breakfaster. So he would probably get better fuel consumption on the way home...

Leslie Brook February 8, 2010 6:37 PM

We also need information on his toilet habits. I regularly lose about a kilo in weight around 8.30am. Would this be before or after Mr Dicksons morning commute. Only he can say...

goodcarlover February 8, 2010 6:38 PM

Me thinks the reason why there is this fuel consumption deficit is that when you go to work you take it easier,when you return home its pedal to  the metal so you have a longer evening in or are going out,maybe a horny g/f wife waiting for you,ha,ha!!!!!

noluddite February 9, 2010 8:13 AM

You surely are joking! To suggest that the mpg is affected by such a small change i altitude over 36 miles. Traffic flow will have an effect orders of magnitude greater than any other factor, unless, as someone has suggested, there is a 20 knot westerly blowing.

LP in Brighton February 9, 2010 9:33 AM

I can well believe the mpg difference is attributable to height difference, especially if it's a gradual slope over many miles.

I used to do a lot of work (for the AA) measuring fuel consumption, and quickly learnt that just about everything has an influence. For example switching the lights or heated screen on, the slipstream effect of other vehicles, even switching the heater to recirculate has a small effect on the Cd!  Also fuel consumption varies roughly with the absolute temperature, so you get about a 1% benefit for every 3 degrees C rise in temperature. I think if every car had a prominent average mpg readout, we'd all learn to drive a bit more efficientlt.    

LP in Brighton February 9, 2010 9:35 AM

I can well believe the mpg difference is attributable to height difference, especially if it's a gradual slope over many miles.

I used to do a lot of work (for the AA) measuring fuel consumption, and quickly learnt that just about everything has an influence. For example switching the lights or heated screen on, the slipstream effect of other vehicles, even switching the heater to recirculate has a small effect on the Cd!  Also fuel consumption varies roughly with the absolute temperature, so you get about a 1% benefit for every 3 degrees C rise in temperature. I think if every car had a prominent average mpg readout, we'd all learn to drive a bit more efficiently.    

coolGav February 9, 2010 12:47 PM

People are more likely to drive efficiently if the car told them a running average of cost per mile of the fuel. If it said it had cost 50p to drive the last mile you might try to reduce that.

Will86 February 9, 2010 1:19 PM

A subtle gradient on a road can have a surprising effect on fuel consumption, at least in my experience anyway. I'm often surprised by the differences in fuel economy travelling in different directions on the same apparently flat stretch of dual carriage way. There is in fact a slight gradient and it easily makes a 15mpg difference. But gradient is only one factor of many.

Will86 February 9, 2010 1:20 PM

coolGav - I think it was on the Gadget Show last night, but there is a system which will tell you the cost of of each journey via a Garmin satnav (I think).

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