Close the Gap


The front of the Cascadia Evolution is aerodynamic. The front of my trailer is not. Together they form a unit. The idea is to get them to be as aerodynamic as practical as a unit. Trailer skirts and trailer tails help. Under tray systems help. They can offer a substantial return on investment. Closing the gap between the truck and trailer does not cost a dime. The only investment is the time it takes to slide the fifth wheel.


I have always heard that 18” is the best distance between the cab extenders and the front of the trailer. I know that if I put my hand right behind my mirror as I drive my truck there is no resistance. As I move my hand back it meets more resistance from the wind. At some point my hand meets a lot of resistance. 18 inches sounds about right. So far I have not held a tape measure out the window as I drive to get it exact.


My last truck had 12 inch extenders. My new one has 20 inch extenders. Even though my fifth wheel is in the same position the extenders let me close that gap. It helps. I drove my old truck 126,000 miles. My best tank was 9.21. My new best is 9.45. The average tank has been about .31 MPG better. This truck has the condo sleeper. That improves the flow over the top of the trailer a little bit. It also has wide based singles that give me a rolling resistance as well as a turning weight loss. Some of that rolling resistance is lost on the new truck because I am running a better traction more rolling resistant Michelin XDN tire as opposed to the XDA that I had on the old truck.


I can't spin the truck quite as sharp as before, but it will turn sharp enough to reverse pivot the trailer axles. That is sharp enough. Moving the fifth wheel forward puts more weight on the steer axle. Almost every state allows 13,000 pounds on your steer axle. Check your axle spec. It should be on the inside of your door. If it is limited to 12,000 pounds, so are you. It is easy for you to check. It is also easy for the DOT to check. Your tires may also limit the amount that you can put on your steer axle. Many tires have a 6,175 pound rating. That limits the amount that you can have on the steer axle to 12,350 pounds. Check your tire ratings. They are on the side of your tires. DOT can also check.


So far I am pleased with the decision to slide my fifth wheel forward and close the gap. I don't notice any difference in the ride. It is a decision that you can make for yourself, unless you have a fixed fifth wheel. Every .1 mpg improvement means about a $685 fuel savings for me. My estimate is that I am saving close to $1,000 per year by closing the gap.


Comments (11)

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 30, 2021


Darren, would it be easier to us H tires with 14,000+ weight rating? I used to have steer tire with a combined 12,350 pound limit. I max out at about the same you do on the steers.

July 17, 2014 20:53:47 PM

Being a set back steer axle heavy truck, this new Evolution is quite easy to over weight the front axle. With full fuel tanks mounted midway on the frame and a typical fry load in the reefer, I can max out at 12780 lbs on the steer axle with my fifth wheel all the way forward, quite a bit over the load limit on my tires. I'm investigating moving my fuel tanks back 6in to better fuel load the tractor while still giving me the ability to minimize the trailer gap.

July 17, 2014 16:39:19 PM

You're welcome Lorence- I am a reformed (LOL) meat hauler. It is the weight of the reefer unit that creates the concern more than the clearance. We almost always would put a single pallet in the nose of the load to try to balance it.

June 15, 2014 7:19:16 AM

Thanks Jeff!

June 15, 2014 1:42:23 AM

Definitely-Kurt and that is definitely a limiting factor. People need to know both there tire ratings and their front axle rating- often you can change the load pattern in the truck. Putting one pallet in the nose of the box instead of two can make a difference. Very easy with odd pallet counts or low pallet counts to put the single in the nose.

June 14, 2014 6:24:49 AM

Finding load range H steer tires out on the road is a challenge, and now with Smartway approved for CA, even more so!

June 13, 2014 18:58:05 PM

A reefer clears the back of the tractor the same way as a van. The 18 inch gap is between the cab extenders and the trailer. It is the corner of the box that comes closer to the tractor. One of the things to consider is the weight of the reefer motor. That puts more weight on the steer axle,

June 12, 2014 5:07:02 AM

Jeff, what if you are pulling a reefer?

June 10, 2014 22:31:11 PM

And every penny you save in your operating cost goes 100% into your profit bucket! That is a $1,000 pay raise for doing next to nothing which is a very smart pay raise.

June 10, 2014 8:13:22 AM

You really closed the gap.

June 09, 2014 6:51:01 AM

This reminds me of England when all we heard was "Mind the Gap". With our straight truck we are able to continue the aerodynamics of our Cascadia all the way through to the back of our truck.

June 09, 2014 6:32:23 AM