Worse than expected economy when towing.

[CheapnCheerful]

I have a Freelander TD4 in good condition which gets 37mpg solo.

I tow a circa 1984 lunar clubman, approx 15 foot over body, max weight 900kg.

Towing I get 22mpg. On comparison with others who get about 37mpg solo, even when towing much heavier twin axles, upper 20's mpg seems to be the norm.

All the usual suspects are known to be OK - correct tyre pressures, brakes not binding, and so on, so I am lead to believe it may be poor van aerodynamics. Although the front is slightly sloped, it does have a deep lip above the windows where the roof joins the front, which cannot help things.

Now as it is a 3 door Freelander, the roof rails go all the way over the back of the car and down the rear pillars. This means, that if I were to fit a wind deflector I would be able to fit it further back than on any other car - it would be possible to have it overhanging the back of the vehicle.

I am thinking of two options.

1. Buy one of the new Purpleline Wind deflectors for £59.99 (I have or can make suitable roof bars) It is claimed that these can save as much as 15% under no doubt well chosen conditions. I suspect the combination I have of poor fuel consumption at present, plus being able to fix it so far back really ought to be able to reproduce the optimum conditions they used, or possibly even better them! At a 15% improvement I would save a tenner or more every trip, meaning payback would be quite rapid.

2. Custom build my own deflector - I could take advantage of the fact that the rails go down the back of the vehicle by cantilevering out the deflector even closer to the van - such that I may have to be careful about clearance issues when on full lock and/or on ramps, and fit deflectors down the sides also. Materials costs would probably be cheaperthan buying ready made, but it would be quite an investment in time, although potential for fuel savings should be even greater. Might be a bit OTT and get a few strange looks though...

 

[xtrailman]

Perhaps reducing speed, and accelerating gently, may help.

I would be expecting 24/27 mpg myself, with your car

 

[nelmo]

That does seem a little low - I get about the same from my Santa Fe but my van is 1500kg.

Unlikely to be one single reason IMO, if you say the car is OK solo - more likely to be related to be a combination of the way you drive (speed, style), the roads you tow on (hilly, m-way/A-roads, b-roads etc).

I wouldn't recommend a wind deflector - I can't believe they can have any useful impact. Any efficiency created by improving windflow would surely be negated by the increased drag and downforce created, no?

 

[RayS]

Agree with my namesake, try dropping your towing speed a little, remembering drag increases as the square of the speed, not linearly. That is, double the speed multiply the drag by 4.

And don't forget that this is airspeed not road speed, so if you are towing at 50 mph indicated into a 10mph headwind the effect is of 60 mph wind. This is particularly important in Uk where prevailing wind is west south west, so if you go to Scotland or lakes and tow back down M6 into the wind (which can be well above 10mph) the consumption will really suffer.

Anothr useful gain is a change of driving stype from solo to towing, whereby you try not to use your brakes at roundabouts etc. but to let the outfit slow down naturally from about half a mile out - you will find your point to point average will not suffer too much but the consumption may well be improved.

Unless you have acess to a wind tunnel, positioning a deflector is largely a hit and miss operation and may well make things worse.

 

[Glenn_5132962]

Agree with the above. Drag squares with speed. If I tow at an indicated 64mph which is a true 60mph according to the tomtom I get about 22mpg out of my combo. If I reduce speed to an indicated 60mph which is a true 56mph my consuption rises to 28mpg.

One of the mags did some tests on a deflector recently & it made the mpg worse!!

 

[CheapnCheerful]

I am aware that dropping speed can make quite a difference. However the main use for the caravan is accomadation at steam shows, which means that it is mostly motorway driving at a constant 60mph (by satnav, approx 64mph speedo). Travel time is more important than usual, as often it is weekend shows and I need to be able to travel 150 miles or more and have time to set up before the show starts, so although slowing down is an option that would undoubtably help, I would only resort to it if there is no other choice. Although the way fuel prices are going it could end up that I am forced to slow as well as adopting other measures...

I will in a couple of weeks be doing exactly the same trip that I recorded 22mpg. All other things being equal, it should show if there really are any significant fuel savings to be made.

 

[xtrailman]

I think if you also take a note of the time taken for the tour, very little time will be lost.

Personally i prefer to just keep in front of the lorries, so i don't hold them up, and they are not constantly overtaking me.

But i also have no problem with allowing the speed to drop off on a long incline, to save fuel.

But i know what you mean, when we use to tour down south, that was a 5 hour tour at night, so the right foot was used to maintain speed.

 

[JD6620]

My Freelander TD4 used to return 37mpg solo and 27mpg towing on motorway in good conditions. But on a tow to Black Horse Farm CC site in Kent a few years back I only got low 20s due to quite a brisk headwind. It does depend a lot on type of road, hills, and wind speed and direction.

 

[Steve L]

First of all, I found that towing our caravan behind our Laguna estate at 60 Mph used far more fuel than dropping down to 55Mph. There is a term for this called over comming the coeficiant of drag. In other words basically you have to go above a certain speed to break up the rolling bow wave of distrubed air produced by the caravan to glide easier through the air. (very complicated and techie auto engineering which is hard to make simple statements from).

Next the fitting of a deflector can save a considerable amount of fuel. I use an original 1970's one bought for

 

[emmerson2]

You could also try "slipstreaming" lorries, but don't get too close! My caravanning pal claims 15mpg from his V8 Disco outfit using this method.You do need patience though. Personally, I'd rather drive at 60ish, and suffer 12mpg in my RR.

 

[CheapnCheerful]

Well I am off tomorrow on the identical trip on which I recorded 22 mpg previously. Rather than go the the expense of buying a wind deflector and finding out it does not help, I have made my own very sturdy one which is approximately the same size as the purpleline one. It is set at 45 degrees and is right at the very back of the car. If it makes a significant difference, then I will spend a little time making a Mk2 version which will also direct the air along the sides of the van as well as over the top, which should give even better results.

 

[CheapnCheerful]

Well I am off tomorrow on the identical trip on which I recorded 22 mpg previously. Rather than go the the expense of buying a wind deflector and finding out it does not help, I have made my own very sturdy one which is approximately the same size as the purpleline one. It is set at 45 degrees and is right at the very back of the car. If it makes a significant difference, then I will spend a little time making a Mk2 version which will also direct the air along the sides of the van as well as over the top, which should give even better results.

Well the results are in!!!

Anyone want to hazard a guess????

Well I will tell you anyway!

With a rough looking piece of plywood, that I all ready had lying around, plus a few other bits and bobs I found to attach it with, for the grand outlay of

 

[CheapnCheerful]

Well the results are in!!!

Anyone want to hazard a guess????

Well I will tell you anyway!

With a rough looking piece of plywood, that I all ready had lying around, plus a few other bits and bobs I found to attach it with, for the grand outlay of

 

[Lutz]

Great, but allow for further monitoring of your fuel consumption before you get too carried away by the positive result. I've had variances of as much as what you've experienced simply due to different wind conditions, all other conditions (speed, traffic density, etc. remaining equal). When towing, a blustry wind can easily make a couple of mpg difference either way.

 

[CheapnCheerful]

Great, but allow for further monitoring of your fuel consumption before you get too carried away by the positive result. I've had variances of as much as what you've experienced simply due to different wind conditions, all other conditions (speed, traffic density, etc. remaining equal). When towing, a blustry wind can easily make a couple of mpg difference either way.

Wind conditions were the same. As far as tests goes this is as accurate as it gets. Identical route, identical loading, virtually identical weather conditions - it was even done on the same weekend at the same time of day as the previous year, so that even the traffic conditions were as near identical as it gets. I even filled up at exactly the same petrol stations, brim full to brim full and calculated using the figures from the reciepts. This was done over a distance of 184 miles. There is absolutely no doubt that a very significant reduction in fuel consumption has occured.

Even if on top of that you alow for a large amount of variance, I feel that there is absolutely no way the savings would be less than 10%. There is no doubt, the results are utterly conclusive that a significant amount of fuel has been saved.

 

[CheapnCheerful]

Wind conditions were the same. As far as tests goes this is as accurate as it gets. Identical route, identical loading, virtually identical weather conditions - it was even done on the same weekend at the same time of day as the previous year, so that even the traffic conditions were as near identical as it gets. I even filled up at exactly the same petrol stations, brim full to brim full and calculated using the figures from the reciepts. This was done over a distance of 184 miles. There is absolutely no doubt that a very significant reduction in fuel consumption has occured.

Even if on top of that you alow for a large amount of variance, I feel that there is absolutely no way the savings would be less than 10%. There is no doubt, the results are utterly conclusive that a significant amount of fuel has been saved.

 

[Lutz]

As was mentioned before, improvements achieved by fitting a wind deflector are a bit of a hit and miss effort due to the lack of information the average driver has regarding the actual air flow around his particular outfit. You can therefore count yourself lucky that you were able to get such a positive result with relatively little effort. That shouldn't disguise the fact that others may not be so lucky or have to go through a long series of trial and error attempts before achieving the same.

 

[CheapnCheerful]

That is certainly true. I think the main reasons why I saw such an improvement is:

1. My outfit was particularly poor aerodynamically to start with, therefore there was the greatest scope for improvement.

2. I had the advantage of being able to place the deflector at the very rear of the car, almost overhanging the back even, something which has been suggested makes a significant difference to the effectiveness of the device.

I think more modern vans are more streamlined to start with and in those cases there might not be such a difference.

 

[ChrisUK]

I cann't see how a wind deflector is going to work unless you can tune your specific rig in a wind tunnel. I'm no aerodynamic expert, but sticking a flap up an the rear of the car seems an unlikely way to gain mpg.

These things were popular 10 - 20 years ago but they faded away as I suppose owners found them of little use.

If the rig is mechanically OK reducing speed is the best solution.

 

[ProfJohnL]

Just to add to Lutz, comment I will recall an experience I had when I did some driving for a company in my student days.

I had to take a Luton bodied Ford Transit with about a 30CWT (shows how long ago it was!) load to Felixstowe from the West Midlands and return with an empty van.

I can't remember the actual MPG figures for each of the legs but I do recall that the difference between the two legs was about 6MPG less on the return (Empty) run!.

All day there had been a NW gale, which helped on the down trip but made it hard work on the return. I also recall that my arms ached from having to continually react to the buffeting from the wind on the lighter van.

 

[CheapnCheerful]

I cann't see how a wind deflector is going to work unless you can tune your specific rig in a wind tunnel. I'm no aerodynamic expert, but sticking a flap up an the rear of the car seems an unlikely way to gain mpg.

These things were popular 10 - 20 years ago but they faded away as I suppose owners found them of little use.

If the rig is mechanically OK reducing speed is the best solution.

So tell me, in what way is an 18% improvement in fuel consumption, in carefully controlled conditions, an unconvincing result? It most certainly has worked!!

 

[CheapnCheerful]

Fortunately, in this case there were no strong winds on either of the runs, so that theory can be discounted.

 

[TD42]

I had one in the old days but as gutters disapeared from cars I could no longer fit it. as i remember the gap beneath the deflecter matterd as much as the position on the roof. It was suggested that when correctly positioned the dead fly/muck line on the front of the van should be approx 18" below the roofline & the gap below the deflector should be less than 2"

 

[Brian_5106957]

I had one in the old days but as gutters disapeared from cars I could no longer fit it. as i remember the gap beneath the deflecter matterd as much as the position on the roof. It was suggested that when correctly positioned the dead fly/muck line on the front of the van should be approx 18" below the roofline & the gap below the deflector should be less than 2"

We recently bought a 2006 Freelander TD4 and have towed our 'van which weighs 1109kg plus load, so probably about 1220kg and I think it drinks the juice -