Fuel Economy Tips

Fuel economy, we all want it but it's not so easy to obtain. We're not driving an aerodynamic 3,000 lb car with a 2 liter engine. We're pushing a 20,000 to 35,000 lb apartment on wheels down the road with the aerodynamics of a 12' tall brick with a huge fuel sucking engine. We're more apt to win the Friends of OPEC award than any sort of energy conservation award. Still, fuel costs money and it's important to minimize it's costs by minimizing it's consumption in any way we can. There are a number of different parameters that affect this, so you will find varying results amongst many RVers, depending upon what they drive and how they drive it.

First, consider that an engine is really nothing more than an air pump. The larger the displacement, the more air it will pump. Also, if we spin this pump faster, it will pump more air. Therefore a smaller displacement engine can pump more air than a larger displacement engine if it spins fast enough. Now, in order to produce horsepower we need to add some fuel to this air because it's pretty tough to burn air by itself. Under normal conditions the fuel and air are at a constant ratio, so they more air we "pump", the more fuel we burn. The secret is to pump as little air as possible.

One of the best ways to do this is to keep the engine RPMs as low as possible. When the transmission downshifts a gear you get more torque for climbing grades. You also get more engine RPM, which negatively impacts fuel economy. If you see a grade coming up, it's more economical to build up speed before the grade by applying enough throttle to add a few MPH without dropping down a gear. Then when you enter the grade, let the speed drop as you climb it by applying enough power to get up the grade without changing down a gear. Once you get over the top let gravity help bring you back up to speed. Truckers call this "letting it drift".

You're probably thinking that a smaller engine will pump less air than a larger one. Well, if the RPMs are the same, that's true. However, smaller engines are suited to pulling smaller vehicles. You may find that a smaller engine does an adequate job pulling a smaller motorhome but doesn't work very well on a heavier vehicle. The smaller engine will constantly be revving higher to get the required power to climb grades and accelerate to cruising speed whereas a larger engine will just lope along at a lower RPM. It's going to depend upon just "how" heavy your RV is and just how small the engine is.

One feature that the Allison transmissions have is a mode switch. When this button is selected the transmission will enter the economy mode. This mode will modify the RPM at which the transmission shifts so that it stays in that gear to a lower RPM rather than downshifting into a lower gear at the normal RPM. This can help keep your revs down but you will notice a bit of speed droop when climbing grades.

Weight does have a factor on fuel economy, but not as much as you might think. If you are lifting weight, it takes some power but if you are pushing it around it's not so bad. Think of a 500 lb block. If you want to lift that block you're going to need some friends. But, if you want to slide it on a concrete driveway you can probably nudge it a bit without too much assistance. Now, place that same block on an ice rink and see how easy it moves. The secret is to reduce the friction between the object, and the surface it rests on. That's why motorhomes have tires that roll. It'd take a whole lot more power to drag the thing down the road on it's frame. Once you get to climbing grades, the weight does enter into the equation more heavily (no pun intended) but on the flat and level it's really a minor element. The more weight rests on the tires, the more friction you have and the more power it will take to roll on down the road but it's still minimal. By keeping the tires properly inflated you'll minimize any rolling resistance, which will help with your fuel economy.

The biggest factor is vehicle speed. Actually, as speed increases, so does rolling resistance, so part of this is weight related. But, the biggest factor is wind resistance. An 8.5' wide by 12.5' tall motorhome with a flat front end needs to move a lot of air out of the way. And, the faster you go, the more air you need to move. Having a tailwind, rather than a headwind, is also a significant contributing factor because driving 60 MPH with a 20 MPH headwind is equivalent to driving at 80 MPH with no headwind. Wind resistance increases with the square of the speed. The following table will help illustrate this.

For the above examples we took frontal area at 106.25 sq ft, which is 8.5' wide by 12.5' tall. In reality, these results are based upon moving a flat plane through the air, which gives us a Coefficient of drag (Cd) of 1.00. In reality, the average bus has a Cd of 0.46 so the far right column should be cut in half for more accurate results. The above results do show that your wind resistance (which is sizeable at 300,000 lbs or more) doubles when the 55 MPH speed is increased to 80 MPH. Seeing as how wind resistance is roughly 70% of the horsepower requirement there can be significant savings just by slowing down. Typical results show that most RVers gain 1 MPG for every 10 MPH drop in speed during highway driving.

Submitted by Mark Quasius - 3/24/06

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