Straight talk from the TL tech team

Expeditious Information

In your January 1998 issue, you printed tow ratings for the 1998 towing vehicles. Since we have recently ordered, but not received, a Ford Expedition with a 5.4-liter V-8 engine and tow package, we were interested in your information. However, a question arose regarding the 16-inch wheels required to reach the specified tow rating. When we ordered our Expedition, the dealership recommended the 17-inch wheels, as they felt the ride of the vehicle would be better. We, of course, want the optimum towing ability but are also interested in as good a ride as possible. Please let us know your reason for requiring the 16inch wheels.

LEON E GORDON

SALEM, OREGON

* Leon, the ratings we published are specified by the vehicle manufacturers who require certain options for maximum towing. Ford rates the four-wheel-drive (4WD) Expedition 5.4-liter to tow trailers up to 7,900 pounds with the 16-inch wheels and, with the 17-inch wheels 7,400 pounds. The gross combination weight ratings (gcwr) are 13,500 and 13,000 pounds, respectively. Shame on your dealer for not providing you with factory tow-rating specifications.

If you look at tow-rating charts, you'll find that gear ratios often affect tow ratings. Larger-diameter tires and wheels reduce the engine speed relative to road speed. This has an effect similar to changing gear ratios and reduces the available pulling power of the engine. Less pulling power equates to a lower tow rating.-K.E

To Tailgate or not to Tailgate?

I do a lot of driving without my trailer Towing the trailer, a 1989 Coachmen, I expect to get bad mileage, but I want to help my truck as much as I can when I'm going solo. I've often heard that I can increase my gas mileage by lowering my tailgate, because this is supposed to let the air coming down off the cab flow through without being caught in the bed. Is this true? I see trucks driving that way now and then.

FRANK LEE

DRAIN, OREGON

* The suggestion about leaving your tailgate down for better fuel economy has been around almost as long as there have been pickups, Frank. In the light of cold hard science, however, it turns out the tailgate up or down issue is probably just a myth, albeit a well-thought-out one.

We recently had a chat with a Ford engineer who reported his company's findings after testing the tailgate theory in a full-size wind tunnel engineering facility, and similar findings were echoed by a GM engineer. Ford tested both the current and last-generation F-150 trucks with the tailgates up and down, with flexible mesh-type tailgate nets, with cab-high truck shells, with flat tonneau-cover-style hard caps and with rooftop-mounted airfoil wings. Keep in mind that neither truck manufacturer was out to prove or disprove these fuel-economy theories as a means of endorsing or condemning any aftermarket parts or claims made by their manufacturers, they simply wanted to scientifically test these ideas and have some hard data available for reference.

In a nutshell, according to both Ford and GM test reports, having the tailgate up or down makes no measurable difference in fuel economy, due to the air flow up and over the cab and down into the bed area. Use of a mesh-type tailgate net, with the wide strap-type fabric mesh, actually decreases the fuel economy because of the air turbulence it creates. Af(at-top "hard" tonneau cover, such as one made from fiberglass, does nothing to measurably increase fuel economy, but a cab-height shell can increase economy because it creates a smoother airflow along the vehicle behind the cab. A GM engineer said that blocking the small space between the back of the cab and the bed, with foam rubber or rolled-up fabric, also can increase fuel economy because it reduces air turbulence created by air flowing through that space.

Curiously, both company spokesmen reported that a hard flat-top tonneau cover that reaches from the tailgate top edge forward about a quarter or a third of the length of the bed, leaving the front part of the bed open, will increase fuel economy.

The Ford engineer said that rooftop airfoil wings work best for fuel economy purposes when tipped up about 20 degrees from horizontal. As for placement, you should be able to stand beside your truck and trailer and visualize the air moving up over the wing, arcing over and back to flow along the top of the trailer If the wing is too far forward, the air turbulates before it reaches the trailer, and if it's too far back, the air can't flow up high enough before it strikes the trailer-JJ.

'Burban Brakes

My wife and I bought a 1994 Suburban 2500, and we tow a 1994 26-foot Sportsmaster from time to time. It seems to be the perfect combination for us. The 'Burb has been very dependable as a family truck. The only problem we are having is the brakes. We bought the 'Burb used with about 32,000 miles on it. At about 40,000 miles, I put a pair of front pads (guaranteed not to squeal) and had the rotors turned. The "no squealing" lasted all of about 20 miles. We talked to the parts store and were told that we didn't break them in right. They said, "Go out and apply the brakes firmly and the pads should not squeal." After about 10,000 miles we took the truck to a repair shop. The pads were worn to the rivets. (My wife and I do not drive with our feet on the brake.) The automotive shop put new EIS brake pads and again had the rotors turned. This lasted all of about another 10,000 miles.

My wife heard grinding noises again, I told her there was no way the brakes were bad again, I pulled off the front wheels and sure enough, the rotors and pads were metal to metal. She took the truck back to the same repair shop and he again changed the brake pads with the same type, but this time the rotors had to be replaced. He also flushed and changed the brake fluid.

We have now put another 5,000 miles on the truck. My wife took the truck to a Discount Tire Center to have it serviced. While it was there, she wanted the brakes checked. The result: front 70 percent remaining and back 60 percent (the back have never been changed).

She called the National Highway Traffic Safety Administration (NHTSA). They told her there was an open investigation on this problem. Have you heard of this before, and is there anything else we can do? As much as we love our Suburban, we cannot afford to keep putting brakes on it every 10,000lS,000 miles.

STEPHEN & HOLLI WIEGAND

MOUNTAIN VIEW CALIFORNIA

* As you mentioned, the original rear brake shoes are still in good conditionthis is a major clue. The rear brakes are self-adjusting drum-types. The adjusters are supposed to operate when the vehicle is backed up and the brakes applied. On these trucks (pickups included), the rear brakeshoe adjusters often don't operate unless the brakes are applied quite hard, sometimes to the point of skidding. Over time, the rear shoes wear, but the adjusters don't move them closer to the drums. This effectively reduces the percentage of rear braking relative to the front brakes and increases the load on the fronts. That leads to premature wear on the front.

I haven't heard of a recall on these brakes, but there are several things you can do that are relatively inexpensive. Inspect the rear brake adjusters for rust and repair or replace as needed. Lubricate the self-adjusters with special brake lube, available in auto-parts stores. Ever since asbestos brake linings were banned, people have reported to be experiencing additional brake problems. Install a goodquality set of carbon-metallic brake linings. These are more durable in heavyduty applications. Make sure your trailer brakes are working well and the controller is adjusted properly. Also, when descending long downgrades, downshift to reduce the load on the brakes, rather than "riding" the pedal.-KE

Micro-Mini Fuel Tank

I have a 1991 Itasca micro-mini built on a Toyota truck 1-ton chassis. I'm interested in buying a larger fuel tank. Do you know of a manufacturer that makes larger tanks for Toyota trucks?

SCOTT GREGORY

SHERIDAN, OREGON

* There are probably aftermarket fuel tanks still available for your micro-mini motorhome, Scott. Several manufacturers who come to mind include Transfer Flow Inc., (916) 893-5209; Rick's Gas Tank Factory Ltd., (800) 267-3482; and RPM, (972) 247-6881. You might also check with the magazines dealing with customized minitrucks or 4WD vehicles, as these publications regularly work with small trucks. Since your motorhome is based on a small truck chassis, one of these magazines may lead you to a source for a fuel tank.

You'll need to watch your loaded weight if you increase fuel capacity. Many, if not most, early micro-mini motorhomes were on the verge of being overloaded as they came from the factory, let alone afteradding cargo. If you up your gasoline capacity by 10 to 15 gallons, that's another 70 to 105 pounds thatyou'll be carrying when the tank is full. That could be pushing your rig's gvwr and gawr to the limits. Check yourweights before choosing a new tank;forewarned is forearmed.-JJ.

Marauis' Marauis

I pull a 23-foot Nomad travel trailer with a 1994 Mercury Grand Marquis LS. In the summer, with the temperatures higher than 100 degrees F in Texas, New Mexico and Arizona, the transmission fluid temperature got up around 230 degrees F, even though I have an auxiliary cooler If I added a wind deflector to the roof of my car, would this reduce the strain on the transmission and lower the temperatures?

WILLIAM E MARQUIS

EL PAso, TEXAS

MA peak transmission-fluid temperature of 230 degrees F while towing in 100-degree weather is normal and is nothing to worry about. If you want added peace of mind, install a larger transmission cooler A wind deflector may improve fuel mileage slightly; I doubt it will improve fluid temperature.-KE

Trailer Tire Swap, Part IV

I'm planning on installing larger-capacity axles on my fifth-wheel trailer and I'm now looking for new 16-inch tires. I've been informed that the only tires authorized for RVs are Goodyear Marathon and Wrangler HT models. The size of the tire I want to use is 235/85 R16, load range E. Can you confirm if I must use one of those two tires?

Alternately, can Michelin LT235/85 R16, load range E, tires, be used instead? I'd like to use these, if possible, since I currently have them on my truck.

LEONA WEIME

VIA E-MAIL

* Those two trailer-specific tire models you mentioned may be recommended for trailer use only, Leona, but that sure doesn't mean those are the only ones you can use on a trailer It just means those tires should not be used on a motor vehicle such as a pickup.

Load capacity and fit are the important considerations when replacing a trailer tire. As long as the wheel is designed to handle the particular tire, you're OK. And, as long as the replacement tire is rated to carry as much load as-or more than-the original equipment tire, it's OK to use on the trailer. In your case, since you're going to a heaver axle and replacing lower-rated 15-inch tires and wheels (you didn't specify the original equipment tire size, but I assume this is the case), then the LT235 Michelins you want to use should be just fine. The primary benefit of trailer (TT) tires is additional resistance to deterioration from the effects of sunlight and ozone.-JJ. TL

Copyright T L Enterprises, Inc. Aug 1998
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