Improve Fuel Economy When Towing

Gas prices are up.  Diesel prices are even higher.  Since I don’t see those prices changing soon, maybe I can DIY some things to help.  Is there any way to improve fuel economy when I’m pulling my trailer?

There are really 2 ways to think about this.  The first is actually improving fuel economy.  The second is finding ways to just spend less on fuel.  They are kind of the same, but they have some distinct differences.  We’ll talk about both.

When we think of economy, we really need to think about what makes our vehicle work harder.  The harder it is to move itself (and the trailer), the more fuel it will use.

What Are The Biggest Fuel Economy Effects?

If we are to improve fuel economy when towing, then we need to understand what makes the biggest difference.  Sometimes it shows the easy solutions, but sometimes the obvious things are hard to change.  Here is a list of things that cause your vehicle to work harder, therefore decrease fuel economy when towing.

  1. The biggest factor in fuel economy is Weight.  It just takes more energy to accelerate and decelerate every time you start and stop and every time up a hill.
  2. Second biggest is Aerodynamics.  This is the wind effect objecting to the vehicle passing through the air.

Some people will swap 1 & 2, so if that suits you better, it’s all good.  I can go either way.  The order can easily change based on the speed and terrain you travel.

We will discuss these in a lot more detail below.

Minor Effects

These next items have a lesser effect, but are worth noting.  These are not so much about ways to improve fuel economy as keeping them from robbing you.

  1. Trailer Stability.  When the trailer is pulling stable and true, there it does not disturb air flow as much.  When the trailer is waggling some, it creates more drag — aerodynamic drag as well as tire scrub drag.
  2. Tires.  Or, more accurately, the combination of tires and air pressure.  Radial tires typically have a slightly lower rolling resistance.  Tires with proper inflation do much better than tires with low pressure.  Consistent pressure for all the trailer tires helps stability, which reduces extra energy output when driving.
  3. Brakes are normally not an issue with fuel economy, unless they drag.  If a brake does not release all the way, it can add resistance — to eat up your fuel economy.
  4. Bearings and Lubrication.  Rolling resistance for bearings is pretty small.  If the bearings are wearing out, or if they are too tight (too much preload), or if you pack them with really sticky grease they can have an effect.  But, with good grease, and good bearings, this is not usually a problem.  (The really sticky, thick greases are good for boat trailers because it helps protect the bearings from water.  Otherwise, use a good synthetic bearing grease.)

For Brakes and Bearings, the best way to check them is to simply jack up the wheel and spin it.  If it spins freely, you’re good.  If it sounds bad (grinding sounds), or quits spinning quickly, then this may indicate something is not as efficient as it should be.  Then, you can dive in a little deeper.

Now let’s look closer at the 2 biggies.

Trailer Weight

When you carry something heavy, it takes more effort.  When you pull something up a hill, it takes a lot more energy.  That’s Weight.  Remember, anything that makes the tow vehicle work harder gives us lower fuel mileage.

So, to improve fuel economy, reduce the weight in the vehicle and in the trailer.  Depending on your situation, RV or Utility Trailer or Car Hauler, you may or may not be able to reduce the weight.  However, any weight you can NOT carry, will help improve the fuel economy.

Every time you start and stop, you have to move the weight.  True, at steady state on flat roads, weight has much less effect.  So, it depends some on where you drive.  Yet, no matter where you go, you will be accelerating at some point, and that is the big consumer of fuel.

We had an old saying in racing.  — If you can’t throw it to the end of the track, and if it doesn’t help everything else get there, then it doesn’t go on the car.  Something to think about.

Aerodynamics

Oh boy !!  This is one of my favorite topics, and there is a ton to talk about.  In engineering school I studied this.  After hours I experimented with it.  On the race course I practiced it.  I really like this topic because it’s not always as simple as we think.

I won’t go into too much here, but if you want more, let me know in the comments, and I’ll consider writing something with more detail.  (In the meantime, check out the many topics at Ecomodder.com )

Let’s start with the equation, in Mathematical terms.
aerodynamic drag equation

OK, that’s nice, but let’s see that again in layman’s terms.

Force of Drag (Wind Resistance) = Frontal Area of the object going through the air, * (multiplied by) a Coefficient of Drag (see below), * (multiplied by) the Velocity Squared.

The mathematical equation includes air density and the 1/2 constant, but we’ll side step those to look at the things where we have some control.

This equation tells us 3 really important things.

1.  The size of the object we are pulling really matters.

This includes both the tow vehicle and the trailer – everything.  If you can reduce the frontal area by half (all other things being equal), then the wind drag is half.  Reducing frontal area is not always possible, but some things to consider are the extras that poke up or out of the trailer, like an RV air conditioner, roof racks filled with stuff, etc..

From an aerodynamic standpoint, you can improve fuel economy by going longer instead of higher or wider.  Again, sometimes it is not practical, but it’s worth knowing.

2.  Coefficient of Drag

Cd For ShapesCoefficient of Drag is a number having to do with the shape of the vehicle (including trailer) passing through the air.  It’s kind of a measure of how slippery the item is as it passes through.  A lower Cd is better.

A football has a lower drag coefficient than a brick, for instance, because of the sharp edges and flat ends on the brick.  For that matter, the football going point first has a lower drag coefficient than the same football going broadside.  We can see that as we compare the Sphere Cd with the Cube Cd in the chart.

Please note that the Coefficient of Drag is NOT about the size.  Size is covered in the discussion above about Frontal Area.  This graphic from Wikipedia shows many differences based on shape.  Not that these are all that applicable to trailers behind a car, but it gives a good perspective.  A sharp cornered front compared to a rounded front makes a difference — but not as much as a nicely tapered tail.  That’s where the real gains are.

Aerodynamics to Improve Fuel Economy
Aerodynamic Trailer Example from Bowlus.

You can reduce the coefficient of drag by removing all the extra things that stick out from your trailer.  Things like bikes up on top create a lot of drag.  Think of it as smoothing the flow of air over the surface of the trailer.

Let’s take a minute to ditch some myths.
  1. While it is true that rounding on the front of the trailer helps reduce drag, the back matters much more than the front.  This has been studied a lot of times.  If you can taper to the back, it will do more for reducing the drag coefficient than a bunch of rounding in the front.
  2. Second, yes, there is a good benefit of the trailer towing in the wake of the tow vehicle.  Kind of like drafting on a bicycle.  However, this does not give the trailer a free ride aerodynamically speaking.  In fact, one area of drag reduction is simply filling the area between the tow vehicle and the trailer.  Here’s an example of a trailer I fiddled with.  This one change of completely filling the void between the truck and trailer made about 0.7 MPG (roughly 5%-6%) difference over a 170 mile trip which I took a few times to compare.  As an experiment only.Improve Fuel Economy
  3. Myth, Under the trailer does not matter.  False.  There is a lot of turbulence under a vehicle, and perhaps especially with a trailer.  The axle, the cross members the tongue beams, a spare tire, jacks, pipes, or whatever hangs down all create a non-smooth bottom which increases drag.  We can argue that because of the ground effect (higher pressure under the vehicle and not wide open for turbulence), these non-smooth things under the trailer are worse there than in other areas.  One simple thing you can do (easier said than done) is cover the bottom of the trailer and make it smooth.  Coroplast (like corrugated plastic cardboard) is a great material for doing this.  Cover the trailer bottom to make it smooth to the wind, and that will reduce aerodynamic drag.
3.  Velocity

Speed kills fuel economy.  In the equation, please note it is Velocity2 (Speed Squared).  That means if you double your speed, you 4 times the aerodynamic drag.  So, the faster you go, the more it will cost.  If you want to improve fuel economy, slow down.  (I hate even typing that because I like speed, but my liking it does not change physics.)  That’s why this is one of the biggest factors to improve fuel economy.

If you are driving into the wind, that effect increases because it’s not the velocity of the vehicle to the road that matters, it’s your velocity into the air.  So, on the good side, if you are traveling with the wind, your velocity through the air is less than the speed of the vehicle with respect to the road.  Well, we can’t always choose to travel in the direction of the wind, but it does improve fuel economy when we can.

As a side note, I had a little Subaru that usually got about 24 MPG.  On one trip traveling in Wyoming into a strong wind, I really had to push the car to get it to go the speed limit.  75 MPH I think.  For that tank of gas, the car only got 13 MPG.  Yes, it was the wind, absolutely, but there is also one more important effect.

4.  Vehicle Controls Systems

All modern vehicles have computer control systems that keep your engine running to perform the task you request.  When you push the gas pedal, the engine responds by providing more power.  In the normal operating range, the vehicle controls keep the combustion close to ideal for efficiency.  However, at heavy throttle conditions (when you are pushing hard on the gas pedal), the control system will often enrich the fuel (put in more fuel) to help control max output and to help control heat.  If they didn’t do that, there is risk of burning up the catalyst or other engine damage.

The enriching scheme provides max power, but it consumes even more fuel.  That is part of why the little Subaru took such a deep cut in fuel mileage on that trip.  The small engine was working really hard to perform as I was requesting, but the downside is it took a lot more fuel to do it.

It is a similar argument for pushing it hard up a long hill.  Driving in the mountains, for instance.  The point here:  When you push the limits of your engine, it will give you all it can, but it will devour the fuel.  It’s a trade-off you have to make.  So, back it off some if you want to improve fuel economy.

Side Note About Bearings (Myth Busting)

If you check out different size axles, you can feel there are differences in bearing drag.  What you are feeling is the difference in size of the bearings and the effect of the grease.  Bigger bearings (bigger in diameter) have bigger pieces inside.  With no load, the grease will have a greater effect on the free spinning.

The other factor . . . bearing preload is usually higher with bigger bearings, which will also give a higher no load resistance.  However, this is a pretty small effect.  Driving down the highway with an appropriate respective load, say 50% of the rated axle capacity, both bearing sizes should roll along with about the same-ish resistance — tiny compared to aerodynamic drag.

As above, if bearings have a lot of drag, it is probably because the have too much pre-load, or old/sticky grease, or they have damage.  Worn bearings will usually make noise like grinding, or feel like they are gritty inside.

Some Out Of The Box Fuel Economy Tips

Concepts to improve fuel economy reach much farther than just while you drive.  Try this list of ways to decrease fuel cost.  These do not change fuel economy, but they do save money at the pump.

  1. We spoke about weight, but how about substitution?  Don’t carry the big vehicle and weight around when you don’t need it.  How about using a Small Trailer Instead of a Truck?  Think about it.  We have a truck to occasionally haul stuff, but we don’t haul stuff every trip.  Some people rarely use the hauling capability of a truck, so why not get a trailer for when you do haul, then leave the trailer at home to enjoy much better fuel economy when you are not hauling?
  2. You can say the same thing about an RV and a Tent.  (Yes, they are different categories, but it’s massive fuel saving, and we have to state the obvious.)
  3. Don’t take that trip.  That might mean skipping the travel, or it might mean don’t go as far.  This seems like a statement of the obvious, but if you drive fewer miles, you will use less fuel.  So, if travel with your RV is what you want to improve fuel economy for, then pick a spot that’s closer.  That will drastically reduce the fuel bill.
  4. Pick your route.  When you do pull the trailer, choose good destinations and stops.  Again, if you reduce the miles you travel, you will reduce the fuel bill.  Isn’t that the point in the first place?

What Can I Do Right Now?

A lot of the things we’ve discussed above are sort of theoretical or require some major overhaul.  That’s nice in perspective, but there are some things you can do right now, today, without a whole lot of work.

  • Reduce Weight.  Just take the things out of your car and trailer that are not necessary.  That might be a little or it might be a lot.  Either way, it helps.
  • Drive Slower.  Just 10 miles per hour less will be noticeable to improve fuel economy.  Even slower will save more.  Sometimes that means choosing a local highway at 55 MPH instead of the Interstate at 75 MPH.
  • Reduce Acceleration and total engine load.  Things like slower accelerations from stop lights, and going slower up the big hills.  Also, coasting more when you may need to stop.
  • Drive Less.  Combine errands.  Choose a closer place to go.  Choose 1 longer vacation instead of 2 short ones.

These 4 things can have an immediate impact on fuel costs.  They may not be practical in some situations, but keeping them in mind will reduce the load on your wallet.

Good Luck as you improve your fuel economy.  The world needs more thoughtful consideration for fuel use.
Have a wonderful day!!

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