The scene from this photo caught my eye. There were 4 dock doors and each one was being used to live load. I was walking in to sweep out my trailer and the scene caught my eye. Three of the trailer gaps were at least 30”. You can see my truck on the end with a 19.5” gap. This little adjustment gives me an aerodynamic advantage.

There are 3 major factors limiting you gap. They are tire rating, axle rating, and weight laws. Every state that I run in allows at least 13,000 pounds on the steer axle. My 16 ply tires have a rating that exceed my steer axle rating. pounds My steer axle has a 12,500 pound rating. That is my limiting factor.

Be careful. Know the limitations of your truck. Most trucks come standard with 14 ply steer tires. These are rated for 6,175 pounds. That limits your steer axle capacity to 12,350 pounds. You may also have a 12,000 pound steer axle. My last truck had a 13,300 pound steer axle. I regularly crossed state scales at near 12,750 pounds legally, and no inspector ever checked. There were several times when I sat on the scale looking at my weights and the scale master held me there – for what seemed like 5 minutes. I always expected them to wander out and check my tires and my axles. They never did. Still, I would not want to be the driver they did check if I wasn’t legal.

My fifth wheel is set to minimize the gap. Once I had it set, I left it there. With 34,000 pounds on my drives, my steer axles sit just under 12,500 pounds. The new longer extensions on my new Cascadia’s Aero X package helped me close the gap. I have not had any issues turning in tight places. When you do turn tight be aware and check your mirrors. My first driving job, the company actually put a bold black strip on all of their trailers. Rule was, as long as you could see the stripe you were OK. Can’t see the strip and you might bend your side extension.

Sometime, put your hand behind your side mirror as you’re driving at highway speed.. Keep the hand close to the mirror and it stays out of the wind. Start pulling your hand straight back from the mirror. At some point the wind will catch your hand and you can feel the pressure. Now, imagine that same force hitting your trailer. That means that your engine has to work harder to pull the trailer. That means more fuel burned, and profits lost.

Comments (2)

Jeff Clark

Jeff Clark of Kewaunee, WI has been driving a truck for 24 years. He has been an owner operator for 11 years.

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April 17, 2018


That is interesting. I had always been taught that the first differential is at 18".

March 26, 2018 16:20:26 PM

100% agree.... and NACFE has built a new part of their website devoted to fixing this area. https://nacfe.org/technology/tractor-gap-devices/

Should you want to close it completely check out our fully automated, deployable gap closer, called the TruckWings. We would be happy to talk to you! www.xstreamtrucking.com

March 26, 2018 11:06:27 AM