Fuel economy difference, flat tray sides on/sides off

Wondering if anyone has a definitive answer to this question.
If you have a flat tray ute, do you get better fuel economy with the dropsides and tailgate off or on the vehicle?
Just been reading the google results and it seems there have been lots of tests done (including a mythbusters one) but they all seem to be on the pickup or ute type vehicle with tailgate down, not removed (think EH or HK Holden type ute with permanent sides and wheel arches).
These tests show somewhat surprisingly that with the tailgate down the fuel consumption increases.
So wondering, to be specific, anyone know about any actual flat tray (no wheel arch type) tests where the comparison is made between no sides or tailgate vs sides and tailgate on.?

When travelling long distances, I take the drop sides off and reckon it improves fuel economy by 5%. It's not possible to do a test really as temperature, humidity, wind, etc all impact on fuel economy but it feels more spritely for sure.

Well….. I'm a fan of that show myth busters.

And as I recall they proved tailgate up being best… probably on the internet somewhere.

Firstly there is a weight factor and it can be reasonably considerable. My tray is 2.25m by 1.8m with 30cm drop sides, that's a lot of steel.

I've done a couple of trips without the drop sides on, one was to Brisbane and back from Melbourne. On the way up I got 10.12 l/100.

On the return trip with a chassis extension and some other stuff that added some metal, plus the missus in the passenger seat, I returned 11.01 l/100 and 10.03 l/100.

The return trip had two tanks of Bio diesel with which I used 11.01 l/100 from the QLD tank and 10.03 l/100 on the second tank picked up in NSW.

There was virtually no difference in speed as I stayed at the posted limit pretty much all the way with the cruise control on and took the same time actual driving both ways. Bit bouncy in some sections with an empty ute.

I think there would be a difference in wild weather and/or if you were moving along at a great rate of knots, but not normally with our low speed driving around Australia. I think with a steel tray and the inherently heavier drop sides, the weight difference could be more of a factor.

Mick.

Edit, I forgot to post my normal highway empty tray with drop sides figures :doh: 10.2 l/100 which is highway speeds usually at the posted limit, as in 100 km/h off the GPS reading, not the speedometer reading.

They always have the gate down on the Drag strip. The sides could not make mush difference as they are edge on to the air flow. Maybe tarp on / tarp off is the deciding factor?

Farmer Geoff. Yeah my mate Lumpy "reckons" the economy is better without tailgate and sides on but the google search said the opposite (however that was for a ute type, wheel arch, vehicle where you cant take the sides off).

Apric tripper. Yes mythbusters did say tailgate up is best.

Optimark. Thanks for your lengthy reply. It would seem you have been wondering the same thing and have even gone a step further and done a few tests. Well done. If I were a scientist (and I isn't) I'd suspect your tests might be lacking a tad in consistency though. eg, chassis extension and missus in front seat one way and not the other.

Hercus.Can't say say I have ever been to the drags. The nearest dragstrip from here is a long way away. Are you referring to utes or flat trays when you say tailgate off? I'd never considered someone would drag a flat tray. Also you are quite right about the tarps! a ute is supposed to get better fuel economy with a tarp/tonneu cover on.

Anyway thanks for the replies.

From what I remember of that myth busters episode…is that tailgate up causes a spin of air inside the whole tray area that helps the flow of air pass over the whole vehicle. Kinda made sense at the time….. And viewers wrote to the show pointing out problems with there initial testing….so they did a 'revisit' . Ended up with similar results. Tailgates up best. Seemed pretty thorough.

Sorry, I didn't read your original post properly. You already read about myth busters. I wouldn't have brought it up if I'd realised that (not taking the . Probably just got distracted and didn't read it properly)

But having said that…..in (I think) the revisit of the myth, it included testing with the tailgate totally removed (what you where talking about ?). I think they also tested some kind of webbing material that sits where a tailgate would sit for those cars that don't have a tailgate, because it improved fuel economy. i.e.…. tailgate up is so believed to improve things in some groups that people have gone so far to produce a product to mimic it presence.

just an opinion. Not an expert at recalling myth busters episodes. 2 cents worth.

They Drag Race both Tray Tops and sedan based Utes. The Tray Tops run with no sides and the Utes tailgate down. Seeing what Myth Busters found out perhaps they should race Gate UP!

Well, interesting I hope to add to this...
I have a 2008 Mazda BT-50 diesel with aluminium flat bed tray. I have been recording my fuel use for 18 months pretty closely, tracking L/km, cost all that stuff.
I drive approximately 110km per day as a commute (55km each way - open road, between 80-110km/hr), unladen.
My long term average fuel consumption is 9.05L/100km. There are a few outliers in this figure, for example when I am carrying a load, passengers etc.
About 2 months ago I removed the sides and tailgate to my ute, thinking it would be more aerodynamic. When removing the outlier (x1) the result has been:
* 9.02L/100km: With tailgate and sides
* 8.99L/100km: Without tailgate and sides
This is not a significant change though.
I am interested by the above comments mentioning an eddy being caused in the tray when the sides are up which acts as a slipstream at speed. I wouldn't have thought it has that much impact. Live and learn.
The experiment continues however....

I can't speak for the sides/tail-gate factor, but on my old Rodeo (w/out sides) I believe I had slightly better economy with a toolbox mounted behind the cab than when it was removed.

It's hard to say definitively, as there're many other factors involved, but the logbooks showed about a 5% difference which I put it down to slightly improved aerodynamics.

Of course, loading up the toolbox reduced any benefits.