Bottom Line
Maintenance Q&A

Paul Abelson
technical editor

Question: Member David W. Etter, Jonesboro, GA, wrote about the gap between the tractor and trailer and its effect on aerodynamics and fuel economy. 

“I am trying to find out what is the proper distance the front of a box or reefer trailer should be from the back of the cab to minimize the wind resistance, therefore increasing the fuel mileage.” 

“My thinking was that the closer the two were, there would be less drag and better air flow over the top of the trailer, allowing the mpg to increase. However, I found out that in my situation, the closer I brought the two together, the worse my fuel mileage got.

“I have posted the same question on the Land Line Forum, under the maintenance section, along with my particular situation and results. However, I have yet to get a definite answer or been able to find out where I could go to find out the answer.

“Any info that you could provide would be greatly appreciated. Since I have to purchase the fuel, I need to get every tenth of a mile or more out of every gallon. And with fuel prices as high as they are these days, every extra mile I can get lowers my cost per mile substantially.”

Answer: According to all I’ve heard at The Technology and Maintenance Council (TMC), the gap should be as close as possible without interfering with trailer swing. I’ve even seen spring-loaded side cab extenders that virtually close the gap but swing away when the trailer turns. Roger Penske runs them on his racecar transporters. The maximum gap should be between 30 inches and 36 inches.

It is surprising that you found mpg dropping when you had the trailer tight. There may be some other drag or some rubbing or friction issues. According to the laws of physics, your mpg should improve, not decrease.

I also mentioned the product I reported on in the last issue, Air Tabs. If you missed it, this is what I wrote to David:

“I should have told you about a product I tested recently, one that impressed me with its simplicity and effectiveness. While many so-called fuel-saving devices operate on secret (or certainly undisclosed) science, Air Tabs use proven and well-known aerodynamic principles. 

“Air Tabs are 3-inch triangular vortex generators, about 1 inch high. They use pressure sensitive adhesive to attach to the rear edges, top and side, of tractors and/or trailers. Lined up side by side, they create small vortexes, spirals of air streaming rearward from the tabs. The vortexes fool the airstreams into acting as if the tractor or trailer had a long, streamlined tail. You can find them at Vortex generators have been used on aircraft since shortly after World War II.

“Air Tabs are mounted on my van. I obviously can’t run a definitive SAE/TMC Type III fuel economy test, but I did find rearward turbulence greatly reduced. The crud that accumulates on rear windows of flat-back vehicles has been all but eliminated from mine. Before I had them, I’d need my rear wiper on continuously whenever it rained or if there was slush on the road. Now, I need about one stroke every five minutes.”

Question: Member Steve Meier, Carterville, IL, wrote to ask, “I want to do more and more of my own repairs, because the more I deal with dealers, the more frustrated I become at paying $90 to $120 an hour for work being performed by factory untrained techs. “I always thought that’s why the labor rates were so high, because these guys were factory trained and you get what you pay for. Boy, was the joke on me. Seems that Pete doesn’t require the mechanics to even be ASE-certified, let alone factory trained. My question: Where can I find do-it-yourself books for Petes?

Answer: ASE certification takes time and experience. Before anyone can even apply to be tested, he or she must know about virtually every problem that can arise in a particular area. Then, certification is done on a component-by-component basis. Engines, drive train, electrical, etc., all require separate testing. 

Most shops recruit from the diesel-tech trade schools and junior colleges. There is a greater need for diesel techs than there is a supply of them. The law of supply and demand is part of the reason labor rates are high. Other reasons involve insurance premiums, benefits, taxes and all the other costs a dealership must pay. 

Factory training takes time, and time is precious. A component supplier like Cummins, Cat, Eaton or Meritor may get around to do training at his dealers once every year or even two. It costs the dealer quite a bit to send a tech to training at the supplier’s location. Besides travel, the tech must be paid for time not spent in the shop. 

Since many techs hop from job to job, dealers want to be sure those they send have proven to be loyal employees who will stay with the dealer over the years. That’s why factory training is usually given to techs who have several years at the shop, not to newcomers. The senior, well-trained techs are supposed to answer tough questions and supervise the newer employees, but they often do not have time.

Labor rates have climbed recently to cover the costs of more expensive major tools for the shop. Also, the tools that techs use are their own. They pay for them themselves. It’s not uncommon for a senior tech to have accumulated more than $10,000 worth of tools.

As far as books, there are books available on general topics (not brand specific) at junior college bookstores, and there are technical manuals for every family of components (Cummins ISX, ISM, N-14, Eaton Fuller 10-speed, 13-speed, etc.) available directly from the suppliers. Since trucks are assembled from suppliers’ components, there is no single manual for a given truck. The Internet is a good source for information. Check for technical bulletins as well as shop manuals. But I think the best book of all is from TMC.

The Recommended Practices Manual from The Technology and Maintenance Council (TMC) has the collective wisdom of almost 50 years of user and supplier experiences, with each and every RP updated every five years or sooner if technology changes more quickly. 

Working on your own truck can save lots of money, but without the proper tools (computerized diagnostics, fuel flow meters, mechanical hoists, air compressors and calibrated air tools, torque wrenches and more), doing your own repairs can do more harm than good. 

Before attempting anything but the more basic maintenance, read through the TMC RP Manual. Even if you decide not to do your own work, you’ll have something to show the techs to help make sure they do the job right. I find it amazing how few techs even know such books exist.

Question: Member Sharon Green, Hegins, PA, wrote asking about an inverter and about using low-voltage appliances in the truck. “We need to know where we can get a low-voltage microwave and toaster oven to run on our 1,000 watt/2,000 peak power inverter that will not burn up our alternator on our truck. We have a 2003 Freightliner Columbia.

Answer: If you have an inverter, you should not need low-voltage appliances. Inverters turn 12-volt direct current into 120-volt alternating current so you can use household appliances. A 900-watt (7.5 amps at 120-volt, 75 amp at 12-volt for short durations) microwave unit should work quite well with your inverter. (Just be sure wiring from the battery to the inverter and the ground is at least 4/0 cable. Then, try an appliance department at Sears, Best Buy, etc.)

Question: Sharon also asked about batteries. “The truck was shut off for maybe five hours, and it didn’t want to start. They (the batteries) were drawn down that far. I had the batteries checked, and they said they were OK. I just had a new alternator put on so I was wondering, are these (Optima) batteries a lot better than regular ones, and if so, which one?”

Answer: Optima batteries look the way they do (like a 6-pack) because they use a new technology called absorbent glass mat (AGM). AGM batteries (also made by Delphi and others) use fiberglass saturated with a gel of battery acid, wrapped up around lead and lead sulfate plates in cylinder form. AGM batteries can be turned upside-down and will never leak. They are powerful and are reported to be long-lived. 

There are three types: starting, deep cycle and general purpose. If you live in the truck and have high current draw when parked, general purpose may be better than the high CCA starting batteries. Your total cold cranking amps (current for 30 seconds at 0 degrees F) should be at least 2,200, preferably up to 2,400.

–Paul Abelson can be reached at

Do you have a maintenance question?
You can write to Paul Abelson, technical editor, in care of Land Line Magazine, PO Box 1000, 1 NW OOIDA Drive, Grain Valley, MO 64029; or you can fax information to (630) 983-7678; or e-mail your question to Please mark your message Attention Maintenance Q&A. Although we won’t be able to publish an answer to all questions in Land Line, we will answer as many as possible.