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Reduced Resistance = Improved Fuel Economy

The travel season is upon us, and once again, fuel prices have begun to climb. Though the cost of fuel has been fluctuating over the past couple of years, one thing you can count on is that it will rise when the flowers begin to bloom. And as you watch the numbers at the pump begin to climb, you may wonder what you can do to increase fuel economy and cut down on travel costs.

You’ve probably seen many articles that have been written about saving fuel, but they typically only touch on things most of us already know, like reducing highway speed and keeping your tires inflated to the right pressure. But there are many other things that can be done to improve fuel economy, and all of them have to do with reducing resistance, in one way or another.

Tire inflation pressure is indeed important, but so is the type of tire that you use on your coach. A tire that is wider than necessary typically produces a better ride, but it can also increase rolling resistance. A narrower, heavy-duty tire will ride rougher than its wide, cushy counterpart, so you’ll have to ask yourself if you’re willing to sacrifice a little bit of comfort for improved fuel economy. I should note here that a larger diameter tire won’t hurt fuel economy and may even help it, because a larger diameter tire turns fewer revolutions to cover the same distance, and engine rpm is lowered as a result.

Alignment is also extremely important. Incorrect alignment can cause a lot of resistance, requiring more throttle input to maintain a given speed. “Toe in” is the most critical setting of them all. Simply put, “toe in” means the front of the tires are “pigeon toed”. Obviously, too much toe will also cause your tires to wear more quickly.

It’s a good idea to have the alignment checked even when the coach is new, because the factory runs them through the alignment procedure pretty fast. And, when the coach is loaded for travel, especially an independent front suspension (IFS) coach, the alignment changes as the load changes. The only exception is self-leveling, air suspension coaches, where correct ride height is constantly maintained.

Straight axle coaches don’t change as much, but it’s still a good idea to check the alignment when loaded. One of the things we always recommend to our customers is that they ask the alignment shop, “When was the last time your equipment was calibrated?” At Henderson’s Line-Up, we measure our alignment jobs in hundredths of an inch. This is important, because if the toe in is just 1/8-inch off spec, that is equal to 28 feet of side scrub per mile—a lot of resistance and tire wear.

Some toe in is necessary when the vehicle is in the static (parked) position, because as the vehicle travels down the road the front wheels will open up, and actually be pointing straight (zero toe in). This happens because most motorhomes have what is known as a “positive scrub radius” on the front axle. On the P32 Chevy/Workhorse chassis, we’ve found that the front alignment needs to be toed in a full ¼ inch at the static setting. The typical alignment shop would think that’s nuts, but we experimented with it, and that’s how much they require to stay at zero toe-in in the dynamic (moving) position. Static settings are important, but I’m more concerned with what the vehicle is doing as it drives down the road.

A lot of RVers don’t know is that the rear axle, or axles, can also be out of alignment—and if that’s the case, this problem can increase resistance and tire wear, reduce fuel economy and cause handling issues. The axle may be askew or not installed properly from the factory—the end result being rear wheel steering.

Have you ever been following a vehicle that looks like it’s going down the road a little sideways? We call that “dog tracking.” The driver keeps the wheel turned to compensate for it, and creates scrub. At the same time, aerodynamic efficiency is reduced, because more surface area is exposed to wind resistance.

Dog tracking isn’t particularly common; I’d say we have to do a rear wheel alignment on one out of 20 coaches, and most of the time they’re off by just a little bit. We perform what is called “thrust angle alignment”, where we put gauges on all four wheels, and set the front wheels in relation to the rear axle. How much affect rear axle misalignment has on the coach depends on the wheelbase; for example, if the rear axle is off by 1/8-inch, the difference in angle will be magnified more over 200 inches than 100 inches, for example.

Trailers can also be affected by incorrect axle alignment. When align a trailer, we line up the axles to the hitch point, whether that’s the hitch ball (travel trailer) or king pin (fifth wheel). That way, the trailer tracks better, and there’s no scrub—so now you’re saving fuel and the trailer tires will last longer.

Now while we’ve been talking about resistance with regard to alignment, there are other factors that increase resistance and adversely affect fuel economy. For example, improving the airflow into and out of the engine is reducing resistance, and usually results in some fuel economy improvement.

There is no “magic bullet” for improving fuel economy in your rig—but a combination of the correct tires and inflation pressure, front/rear alignment, driving style and engine enhancements can add up to substantial savings each year.