Riddle me this...

Is the tool box new?

No but it used to have legs.

Tested it with and without trailer at a constant 55 miles an hour on a flat paved road.

Trailer only weighs 350 pounds and has trailer tires with 55 pounds of air.

(I can push the trailer at a run by itself with it's front wheel stand down on pavement. Very little rolling resistance.)

The car gets 31 mpg without the trailer.

The car gets 32 mpg pulling the empty trailer as shown.

Car has an mpg readout that constantly updates current actual mpg and is pretty accurate according to the old 'fill it up, drive it, fill it up again and divide mileage by gallons' method.

(Not sure if mpg readout accuracy is important since it is a relative measurement here...change in mpg with/without trailer.)

My first guess would be that after installing the hitch, you checked the air in the car tires and added some air to one or more tires.

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The anomaly might be due to difference in conditions. Did you actually drive the same course without the trailer just before or after you drove with the trailer? Warmer tires can result in a noticeable improvement. Air temperatures can also have an effect.

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A contributing factor could be the precision of the mpg read out and the different variables that could have an effect. The improvement is odd, but it might be a combination of normal variability in the MPG readout and slightly different driving habits and conditions.

Tongue weight is 50# with the trailer empty.

Didn't change the tire pressure.

Same road, to work and back. All roads here are nearly perfectly flat except for slight undulations for drainage. Our only 'hills' are overpasses, lol.

(I live on the edge of the Everglades so the whole world is flat here as far as the eye can see which is a hundred miles sometimes, literally.)

Cold start both times, drive 23 miles out, work, 23 miles back.

MPG was maximum continuous observed on cruise control in approximately 5 mile segments.

My Dad kept a 2"x3" spiral notebook in his glove compartment and recorded the mileage of every tank of gas he ever bought I think so I am a mpg nut, born and bred. Not as bad as he was but he was a Depression Era kid.

That is puzzling.

You might try filling up just prior and filling up again just after your drive to work, then doing the same without the trailer. Temperatures are important, so logging temperture for each leg of the trip would be a good idea.

Having a job working through your data, here. Do you have figures for Vibe with and without trailer and TT with and without trailer?

So, if you put ten trailers behind it, you will get an amazing 41 MPG!

I think that over time, you will find slightly decreased, (average), mileage with the trailer.

It could be explained by simply having a tailwind at your back.

My 86 GMC truck gets a consistent 10-12 MPG...it doesn't matter if it's loaded with concrete or empty. It doesn't get much road time anymore.

Yeah, my 2000 Silverado gets a constant 14 mpg loaded all the time with 500# of tools in the bed and a 250# fiberglass topper.

(Actually, the TT does get 41 mpg going downhill on overpasses, lol.)

Drive was out and back with turns so wind was variable, mpg was consistant.

............................................

Vibe without trailer...31 mpg.

Vibe with empty trailer...27.5 mpg. (But toolbox was raised up on 16" tall legs.)

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TT without trailer...31 mpg.

TT with empty trailer...32 mpg. (Tool box lowered and bolted securely to bed and tongue.)

Trailer is so light, 350# and high pressure trailer tires, you can barely tell you are pulling it.

Might be that the program in the ECM in the TT is so sophisticated compared to the one in the Vibe that it senses the load and changes the fuel/timing/torque values from 'performance' to 'efficient'?

This would make me think it would compensate a little bit for perceived load so perhaps the car is smarter than I am. :)-

Audi's manual is ambivalent about towing, different specs in Europe and US.

I'll load all my dogs in it this weekend, #250, without the trailer, and see if the extra weight decreases mpg.

If it does, then the trailer is 'drafting' me and creating less drag.

You can't draft yourself.

'...You can't draft yourself.....'

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That's the problem with snappy one-liners. They feel right, and people have a bias towards being swayed by emotion.

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You realize that drafting isn't just of benefit to the vehicle in the rear, right?

When two cars or bicycles are drafting, the pair can often attain higher top speed than when not drafting. This would not be possible if the there were not benefits for the lead as well as the follower.

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All that is needed is a connection and for the reduction in drag to exceed what is needed to pull the trailer.

You still can't draft yourself.

You have just become too reliant on CADD.

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It may take longer, but you can do it all by yourself.

Wish I had CADD. . The principle of drafting is that when a number of powered vehicles are travelling in convoy, they may achieve a higher top speed for a given power, or a reduction in COMBINED fuel consumption, for a given speed. . So long as nobody goes to sleep. ;-) . "That's the problem with snappy one-liners. They feel right, and people have a bias towards being swayed by emotion." . It also helps when they are right. . "You realize that drafting isn't just of benefit to the vehicle in the rear, right? . Correct, I didn't say that. . "When two cars or bicycles are drafting, the pair can often attain higher top speed than when not drafting. This would not be possible if the there were not benefits for the lead as well as the follower." . Yes. . "All that is needed is a connection and for the reduction in drag to exceed what is needed to pull the trailer." . Yes, but in this case* it is an aerodynamic effect or something else, not drafting - we only have one driving vehicle. . *Taking OP at face value.

You can't draft yourself.

Drafting is not an aerodynamic effect?!?

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Consider the following:

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Okay two cars are drafting with very little separation. .... (I'm just going to assume that you agree the cars are drafting, since it is a given in the scenario)...

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The rear car then uses a robotic clamp to grab on to the trailer hitch of the car ahead. The speed nor power provided by each car has not changed as of yet, so the rear car is not pushing nor being pulled by the car in front. ..... Is this still drafting?

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Then the front car begins to provide more of the power and the rear car provides less in a synchronized fashion, so that the speed remains constant. ....When the front car is providing all the power, is this still drafting? If not, when did it cease to be drafting?

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Two bicyclists are riding with one very close behind the other. The slope is slightly downhill such that the rear cyclist does not have to pedal. If their tires momentarily touch.... Are they still drafting? What if the rear cyclist reaches forward and grabs the brake boss on the front cycle even though no pushing or pulling occurs, are they still drafting?

"Drafting is not an aerodynamic effect?!?".

Absolutely it is. The rear car reduces the tail vacuum on the lead car, the lead car reduces the head pressure on the rear car. Both benefit.

"Consider the following:"

Possibly next month.

'Drafting is not an aerodynamic effect?!?'

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....was my statement of disbelief at your insinuation:

'...Yes, but in this case* it is an aerodynamic effect or something else, not drafting...'

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You seem to now be stating outright that it is an aerodynamic effect (which it is)... what did you mean in your immediately previous comment?

I probably need a clear definition of the engineering term 'drafting' then in order to find the correct term.

Perhaps you could suggest one?

I was just using the general term used by laymen when one vehicle follows so closely to the vehicle in front of it that a large portion of the engine power normally used to maintain a highway speed is no longer necessary to maintain said speed.

I sandwiched myself in my '64 TR-4 one Christmas vacation between two semi-tractor trailer rigs doing 80 all the way from Tampa to Atlanta and got 5 mpg more than I normally got going 80.

All I could see was bumper out the front and a huge Mack grille in my rear view mirror but we all three probably saved some fuel that trip. I know I did.

Luckily I didn't end up as road kill!

To me "the engineering term 'drafting'" means making lines on a piece of paper that can be used to define a mechanical item.

This is the Transportation Section though, so applying the term automotively, I would infer that your definition of drafting would be accurate, given the context.

It would appear that there are many definitions for the word draft!
Perhaps 'slipstreaming' would be better but that precludes the effect on the car and only explains the effect on the trailer's reduction in wind resistance.
http://dictionary.reference.com/browse/drafting
I am currently headed to the fridge for some drafting now...

Watching NASCAR, Danica didn't make it through the first lap without wrecking.

She won't be doing any drafting today.

Your use of 'drafting' was not confusing for me.

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The only thing that I would add to your just stated general term definition: something explaining that there is often benefit for the lead vehicle in addition to the following vehicle.

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This is important, because one might think the following vehicle is simply 'mooching' off the lead vehicle, which isn't really necessarily the case.

'Drafting' is an anticipated reduction in fuel consumption, per vehicle, when traveling in close convoy, as part of the aerodynamic load is shared. This case is an unanticipated reduction in fuel consumption when the (apparent) aerodynamic load is increased, for effectively a single vehicle.

Don't tell Ford that. (Myth Busters test of a Ford truck MPG with tailgate up or tailgate down, controlled conditions, tailgate up won) I suspect the same phenomenon here. -- JHF

Beats me.

I can't think of any logical explanation for pulling a trailer, increasing gas mileage.

Unless....

What if the extra weight is somehow causing your computer to lean down the mix, change your timing, etc., which is causing a slight increase in fuel efficiency?

This is cool...

http://www.youtube.com/watch?v=Mmhmt2OLNtA

and this

http://www.youtube.com/watch?v=nYmB3C-r-_s

There might be something to this.

I've seen studies that say taking the tail gate off pickups decreases mileage and semi-tractors all have roof top air dams now.

Put the legs back on the box and test it that way.

There's a lot of stuff going on there...if yours is like this.

http://www.audiworld.com/news/02/tt32/

I can't help but wonder if your car can "sense" that you are in tow mode, and not in a race, and adjusts the engine and transmission accordingly.

Aggree your car's tranny and engine aren't designed for towing. Watch out or you'll be replacing both.

Perhaps the 50 pound tongue weight causes a shift in height of the car,making it ride lower,and become more aerodynamic.

Is there a difference in the tire air pressure from front to rear? If so, taking weight off of the front by applying downward pressure on the rear could reduce rolling friction of the front tires.The additional weight on the rear tires may not result in an exact increase in rolling friction of the rear tires,as compared to the front.

Just guessing at this point.

I would advise adding a transmission oil cooler if you intend to use it for towing regularly.

It's a stick shift 6-speed and the car didn't move at all with the 50# tongue weight.

Very stiff suspension to begin with since it is a 2+2 seating configuration.

I'm liking this car more and more every day since I bought it 3 weeks ago.

Actually, mine was made in Feb.2000 and sold in the USA as a 2001 model which had the spoiler on the back to keep it from lifting at triple digits speeds.

I forgot to mention it is the 6-speed Quattro model and has a 700# weight capacity.

It is mentioned in the literature that one cannot tow with the base front wheel drive model but the Quattro model with the Haldex will do it.

I'm pretty certain that cruising at 70 mph pulling a 350 pound trailer results in very little strain on the drive train compared to kicking in the turbo with 15 pounds of boost and 207 foot pounds of torque at WOT in the first three gears.

I have put AMSOIL Severe Gear 70W-90 lube in the tranny and rear end already after reading this study but that was after the tow test.

http://www.goodsenseoil.com/G2457-GearOilWhitePaper.pdf

What's the weight of the bikes being towed on the trailer?

There is the possibility that it acts as a spoiler, disrupting the pressure reduction that is normal at the back of any boxy type vehicle. Without it, the suction acts on the whole of the back, but only on part of it when the trailer is there.

I'm thinking that too. The solar racing vehicles are shaped like a flattened out tear drop and my trailer bed is only 2" thick so basically it extends the perceived rear of the car back another 12 feet. Much like this design does...

That actually makes sense. Especially if you consider how a KF airfoil works (which might explain why the square backside of the tool box isn't an efficiency killer).

Hmmm...so if the car has 4.5" of ground clearance and the trailer has 8" of ground clearance what I have is a KMf-4 profile which should create some lift on the trailer making it lighter the faster I go?

This is going to be an interesting trip to the mountains next week.

I'm pretty sure the motorcycles will take care of the lift, one way, or another.

I know, I was just curious about the results of the empty trailer test.

With a load and the added frontal area of the 'naked' bikes all benefits of the KF effects will be gone with the wind.

Pardon me for being a bit too blonde, but ... Considering the reason for the car-trailer assembly in the first place, wouldn't it be more worthwhile to put the bikes on the trailer before you pulled it 50 miles?

Doh!!!

That explains the 1 mpg increase...

He forgot the bikes.

Actually I was testing the hitch installation I had done myself. I bought the hitch used from a guy who sold his TT and he said he had no problems at all.

He did fail to mention that you had to duct tape the mounting bolts to your finger to install them on the driver's side by way of inserting them and your finger into a 1" diameter hole in the frame and then out the smaller bolt hole and then through the hole in the hitch mounting plate.

Took me an hour to get the first one in without the duct tape and 4 seconds to do the other one.

A Cali girl can never be too blond!

What is the big deal in this. I consistently get 49 mpg on my Suzuki - Wagon R (although no trailer) at 62 mph.

Anil

This isn't about bragging about fuel economy.

The 'big deal' is that an increase in economy is not what is the expected result of towing a trailer.

Two data points don't make a trend. Assuming you drove exactly the same route, there are many other factors that could make this small difference (3%) such as wind, air temperature, other traffic, tire pressure, how accurate the shut off is on the gas pump, etc. To be statistically significant, this test would need to be repeated a number of times.

I agree with what someone else suggested...put the legs back on the toolbox and see what happens. Nothing like an empirical study to gather data! I suspect you will see a reduction in mileage.

I have to say that I "feel" that the toolbox could be a part of the problem, you need to try it over a test "circuit" and record the results.

You need a test circuit that has a road, as long as possible, a) with toolbox, b) without toolbox and compare them.

Use weights equal to the toolbox weight to stop weight changes caused by removal of the box affecting the results.

I would also strongly recommend blowing the tyres up to the legal max pressure at all times....this reduces tyre resistance and the heating effects when driving and when fully loaded, improves braking if a braked trailer...the same for the towing vehicle.

If all else fails, write to MythBusters!

There are may things that can explain this as I found out when I was the owner of a dyno tuning shop. As Lyn mentioned I have seen at least two studies done by universities that showed dropping the tailgate on a pickup lost up to 17% milage mostly due to lift being created in the box causing parasitic loss. Tonneau covers and toppers almost always save fuel with streamlining. Some engines are more efficient under some load conditions. The ones I remember best were the Ford Pickup's with the 360 ci. engines. Almost always they got better mileage with heavy loads such as one customer that had a large flatbed trailer and hauled 4 atv vehicles and all the supplies for a week to northern Ontario hunting moose. The tool box may very well get rid of a negative pressure at the back of the car that is causing some higher than usual drag enough to overcome the effect of the trailer in general. I have seen engine temp cause an increase in fuel savings such as I experienced with my Ford 300 ci. in line six. Next thing is to do the test with the bikes on the trailer. In the mean time enjoy the improvement as the variables are as many as the commentators.

I also have a vehicle (Vixen Motorhome 21TD) (See vixenrv.org) that gets better mileage when I tow my Suzuki Samurai behind it than when I drive with only the motorhome. The Vixen is famed for it's mileage and was designed to get 32 MPG at 55 but who drives 55? I get 24-25 when I keep the speed below 75 without the Samurai behind, but this increases to 26-27 when I tow the Samurai. I have a fantasy project to build up a hybrid version of this vehicle on paper only and have studied the reason for the increase. In this case it is the low pressure area imeadiately behind the motorhome that causes the problem with the aerodynamics of the motorhome. The Samurai stirs up the air and breaks down the low pressure. It is obvious from looking at the back of the car you are using that it will produce a low pressure behind your car. The trailer disturbs it. If you designed the right type of spoiler on the back of you car, you could probably get better mileage than towing since the trailer has a lot of drag in itself. I know that my Prius has a "spoiler" on the back because they called it that and charged extra. It is very subtle and hard to tell setting side by side with a standard spoiler, but it adds about 2MPG to the vehicle.

BTW, the best I get driving the Samurai by itself is 25mpg.

Wow, your Vixen is a rare bird! Only 587 of them were built. I've seen one on I-75 before but didn't know what it was.

Samurai's are boxy too but maybe the small cross section helped to smooth the flow.

It could be what you have done is found a close cousin to a perpetual motion machine, the air flow, coming off the lead vehicle, curls around and PUSHES the tool box forward!

It does get 200 mpg going downhill in neutral at 70 mph!

The Toolbox works like a winglet? That is the only thing i can think off.

I'm pretty certain Truth's answer in response #27 is the right one because it eliminates all the turbulence on the back of the car.

I will be really happy to get 30 mpg pulling it. I'm hauling some screen doors and an air conditioner up and bringing my '96 MZ 660 thumper back so I'll have some different loads to compare.

Slip-streaming or drafting (a term I have never heard of before with regard to following a vehicle closely), was explained to me many, many years ago on a BBC TV Program.

It appears that every vehicle, on its own, has two areas of wind resistance (my terminology may be a bit rough, sorry!).

The first one is the air in front of the vehicle that needs to be "pushed aside", the second is the vacuum and turbulence left behind the vehicle, that effectively "sucks" the vehicle backwards.

But, when slip streaming, the front vehicle only has to push the air at the front, out of the way. The following vehicle has basically a vacuum "hole" to drive in and does not need to push much air out of the way the front vehicle is not sucked back as the following vehicle has taken it over so to say, and the rear vehicle only has the rear turbulence to handle.

Basically the total wind resistance of all types for a single vehicle are halved and each of the two vehicles saves around 50% of the total wind resistance, though generally speaking, the rear vehicle gets the better deal in my experience.....

Race cars make good use of this effect for overtaking, but lose the efficiency of the down force of spoilers when following, due to the reduced density of the air behind the front vehicle. Through this, many Formula one cars have actually accidentally hit the front car when braking.

I found some links which may prove interesting:-

http://www.formula1-dictionary.net/slipstreaming.html

http://en.wikipedia.org/wiki/Slipstream

The world record push bike top speeds were made behind a fast moving locomotive if I remember correctly, with planks laid between the rails for the bike!

There is an interesting dissertation on this subject here:-

http://en.wikipedia.org/wiki/Cycling_records#History_of_motor-paced_records

As Truth stated in response #27, I think maybe the 'vacuum' behind the trailer has been eliminated completely because the 'wind' sees a KF airfoil now with a bubble behind the tool box and since the trailer is very thin like an airplane wing it allows the envelope to close smoothly.

I'm sure if I load up the trailer the turbulence will return.

Might be a good business to fabricate lightweight storage boxes in the shape of the tapered end of a teardrop that would fit into a standard trailer hitch so that the extra weight of the cargo carried is offset by the reduction in drag.

Need more trials. So far, you have one data point for loaded, one for unloaded, correct? Without seeing your data and the design of the experiment, I would hesitate to say this is anything other than statistical noise. Write it up for me and I will review it.

Besides, didn't the dealer tell you that your mileage may vary?

Driving 700 miles tomorrow...will get back to you soon.

Perhaps the taper of the 2 vehicles combined results in a lower overall drag than what would be the case for the single vehicle. The addition of the trailer lengthens the aerodynamic envelope, and its profile does not end abruptly as does the car by comparison. If true, the answer to the general question " is drafting helping me " would be YES. Would be interesting to see a wind tunnel or other analysis.

I also wonder if the monohull yachting W.L.L.* rule comes into effect here as well, theoretically yes, but in reality?

* = Water Line Length rule.

Only if it is raining really, really hard.

LOL!!

A more significant factor may be the difference in air pressure/temperature on the two days.

Another factor might be getting used to the vehicle affecting driving style.

I have observed that engines have a unique "sweet spot" where efficiency is greatest.This is not necessarily the least throttle opening or slowest speed.

While travelling through mountainous terrain,going uphill on a long,slow grade, my Accord did not need to downshift as long as I held the RPM at 2500 rpm,or around 72 mph.

Anything below this would result in a downshift, and increased RPM and lower milage.

This effect was observed in multiple coast-to coast trips over varying seasons and weather conditions.

A wider-open throttle reduces engine losses,although consuming more fuel.There is a point where the two lines intersect, and that will vary by vehicle.

A mileage contest in the 1970's was won by a Ford Pinto, and the driver used a very unusual technique:He started in 2nd gear, and held the throttle wide open up to 30 mph, then turned the engine off, then coasted down to 10 mph,put it back in gear,let inertia restart the engine, with throttle wide open,and repeated the procedure the whole test.His method surpassed all the other ones used at the time.His explanation was a reduction in pumping losses because of the wide open throttle.

Perhaps pulling the trailer loads the engine to a "sweet spot" of the engine's torque/horsepower curve.I am sure this may be only one of many factors that combine in your case,including the aforementioned effects of the much maligned term"drafting".

Please keep us updated as you perform further testing.

HiTek "His explanation was a reduction in pumping losses because of the wide open throttle." Also, it would be due to no pumping losses when coasting - I imagine he got a fair ratio of coasting to 'W.O.T'.

According to my Dad who was a car nut to the point of having his own used car lot, all cars have a 'sweet spot'. He would always ask me after driving my many cars from Fla to Indiana for Christmas, "Where does it like to run?" meaning where was the 'sweet spot'.

Easy to find it because you will simply need the least amount of gas pedal to run at that speed. Very often in my GM trucks it was at 72 mph but they all had small block Chevy's and 3.73 differential gears.

My GT-6 liked 90 but that was back in the days before the 55 mph Interstate speed limit.

My current 2000 Z-71 likes 86 mph.

The TT seems to be happy anywhere but it has a turbo and 91 of intake plenum, 5 valves per cylinder and who knows what else, lol.

I think it is a matter of torque/hp curve, MAF, fuel octane, tire pressure, and load. Weather never seemed to be a factor from 0-100F degrees.

Maximum torque with minimum horsepower would seem the logical point of highest efficiency.

Shell High Test always gave me the best mileage and still does.

OK...after 3 700 miles, non-stop, trips, one with the trailer gross weight of 650 pounds one with the trailer empty and one without the trailer gave these results.

The trailer weighs 400 pounds empty and has a single axle, 55# of air in the tires.

During all three trips the car (2001 Audi TT Quattro 225hp, 6-speed standard tranny) was loaded with 180# driver and 4 dogs weighing a total of 250# plus 8-16 gallons of gasoline, and 30# of gear.

Tire pressure was NOT changed for any of the three trips, 36# front and rear.

Car normally gets 32 miles per gallon, full tank of gas, driver only.

Average speed 72 mph over 700 miles (600 on I-75).

Loaded trailer/loaded car....24.6 miles to the gallon

Empty trailer/loaded car.....25.6 miles to the gallon

No trailer/loaded car..........28.5 miles to the gallon

Return trip next week will be with loaded car, increased tire pressure to 40# front and rear. Hoping to get 30 mpg loaded.

Off topic note: My 2003 Pontiac Vibe (with standard 5-speed, 150 hp, variable valve timing) with the car loaded with 600# gear and trailer with two motorcycles and full riding gear and tools (1200#) repeatedly got 27 miles per gallon at 72 mph on I-75. This would be the optimum package for me for adventure trips if they made it in AWD with a standard tranny but they don't!

However, the Audi TT's FUN FACTOR is off the charts so I will be keeping it. :)-

Just came across this (talking about a "robot truck" slipstreaming a human driven one), for anyone still interested:

Besides safety, the major selling point of this system is that the reduced drag saves fuel costs. Peloton says the "technology saves more than 7% [of fuel] at 65mph - 10% for the rear truck and 4.5% for the lead truck," ....... These savings come primarily from reduced aerodynamic drag.

Here.