Minimize Resistance, Maximize Fuel Economy

June 16, 2008

There’s no question that fuel economy is at the top of every RVer’s list, so in the next few posts, I’m going to talk a little bit about ways to improve fuel economy. I’m not speaking of the typical suggestions, such as driving 55 mph or slower or monitoring tire pressure (although those do help), I’m talking about ways to reduce resistance overall. Reducing resistance improves efficiency, and that means better fuel economy.

Obviously, tire inflation pressure is 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. That’s why a lot of truckers are going to what is called a “super single” on the rear of their trailers; instead of duals, they run one very heavy-rated tire and wheel per side. Our shop hasn’t experimented with this, and the motorhome industry hasn’t introduced super singles yet, but it could come to that. 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. This is a pretty broad subject, which is why we’re not going to try to cover it all in one post. 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. Here’s a little graphic of what toe in/toe out looks like:

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, except on 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?” 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. We want the toe in to be as close to zero as possible when the vehicle is on the highway to assure the best possible fuel economy and tire life.

In the next post, we’ll talk about other alignment settings as well as rear axle alignment and trailer axle alignment. Happy travels!