Trailer side skirts do save fuel, but only a few percent, and mostly at steady highway speed. On long interstate runs that can add up to real money. In short city work, the savings shrink and the downsides stay.
Here is how they work, what the savings look like in dollars, and where they cause trouble.
What is a side skirt on a trailer, and what does it do?
A side skirt is a panel mounted along the lower side of a trailer. On a dry van it runs between the landing gear (the legs that hold up the front of a parked trailer) and the rear tandem axles. It is one of the standard aerodynamic add-ons described in EPA SmartWay and DOE/NACFE trailer material.
The job is simple: close off the open gap under the trailer. Without the panel, that gap is a big hole in the side of the rig.
Why is the air under a trailer such a problem?
At highway speed, wind does not only hit the front of your rig. Crosswinds and the airflow along the sides push air into the space under the trailer. It tumbles around the axles, wheels, landing gear and frame.
That churning air is turbulence, and turbulence is drag. Drag is the air pushing back against your truck, and your engine burns diesel to overcome it. The underside of a trailer is one of the messiest places for air on the whole rig.
A skirt blocks much of that air from getting under there. It helps the air stay along the side of the trailer and slide past, instead of tumbling around the running gear.
How much fuel do skirts actually save?
The honest answer is a few percent at highway speed. NACFE and DOE reporting consistently puts skirt savings in the low single digits. Results vary by skirt design, by speed, and by wind conditions.
EPA SmartWay runs a program that verifies trailer aerodynamic devices, including skirts. Verified devices are sorted into tiers by how much fuel they save. If a skirt is SmartWay verified, it has been tested and meets a fuel-savings level. A product with no verification is only making a claim.
If you want to know what a specific skirt delivers, look at its verification. Don't rely on the sales pitch.
What does that look like in dollars?
Here is an example. These numbers are illustrative assumptions, not measured results, so swap in your own.
- Say you run 100,000 highway miles a year.
- Say you average 6.5 miles per gallon.
- That burns about 15,385 gallons.
- A 3 percent savings is about 462 gallons.
- At $4 a gallon, that is roughly $1,800 a year.
If your mpg is better, you burn less fuel to begin with, so a percentage saves fewer gallons. If your fuel costs more, the same percentage is worth more. If your skirts save less than 3 percent, scale it down.
The point is that the savings are real but not magic. They are a steady trickle that adds up over many miles. They won't pay for themselves on a truck that mostly sits or crawls.
Why do skirts pay off on the highway and not in town?
Aerodynamic drag grows with the square of your speed. Double your speed and drag goes up about four times. That is why the same panel does far more work at sustained highway speed than it does at 25 miles per hour.
In stop-and-go driving, the engine is fighting weight and acceleration far more than wind. There is little air resistance to cut, so there is little fuel to save. EPA SmartWay notes that the benefit is mainly at highway speed.
So the lane you run matters:
- Long-haul runs at steady highway speed get the most benefit.
- Mixed regional work gets some benefit, depending on how much is open road.
- Short city and dock-to-dock work gets much less.
What is the catch with skirts?
Skirts hang low, and low things get hit. They can scrape or get damaged on steep dock approaches and on humped rail crossings, where the road rises in the middle like a hill. Snow, ice piles, curbs and rough yards can also catch them.
Brackets and panels cost money to repair or replace, and a trailer that is down for skirt repairs earns nothing. Skirts also add weight to the trailer. The amount depends on the design, so check the spec for yours. On a load that is close to its legal weight limit, extra pounds on the trailer mean fewer pounds of freight.
Some designs are made to flex or fold up when they hit something, which helps. Even so, they are not damage-proof.
How do I protect skirts and get the most out of them?
A few habits go a long way:
- Take steep docks and humped rail crossings slowly and straight on. An angled approach drags one side's skirt into the hump first.
- Look at the skirts on your walkaround. Check for cracked panels, loose or bent brackets, and anything hanging or dragging.
- Report damage early. A small crack or loose bracket can turn into a lost panel at highway speed.
- Watch the ground clearance when pulling into unfamiliar yards. Low skirts do not forgive a bad approach.
Who pays for skirt damage depends on who owns the trailer and what your company's policy says. That is worth knowing before you scrape one.
Are there rules about skirts I should know about?
There is no general federal requirement to run skirts on your trailer. Some states and provinces handle aerodynamic equipment differently, though. California, for example, has its own rules that push certain trailers toward SmartWay-verified aerodynamic equipment. Canadian provinces also set their own rules on trailer length and equipment.
Because the details change, check the current rules for the places you run. Your carrier's safety or compliance department should know what applies to your equipment.
So, are skirts worth it or just a headache?
On long highway runs, skirts can earn their keep. A few percent of a big fuel bill is real money, and it keeps coming in mile after mile. In short city work, much less.
The headache part is real too, mostly in the form of scrapes, repairs and weight. Take docks and crossings with care, and give the skirts a look on every walkaround.