February 1, 2005

TT #42 Improving Fuel Economy Part 1 - Gas Engines

By: Rob Lowe

Technical Tips #42 


Tips to Improve your Fuel Economy

With the recent rise in the price of fuel, many have become concerned with the increased cost of RV travel. The reality is that fuel is a relatively small part of the overall cost of owning a RV. The typical RVers travels less than 12,000 miles per year based on a recent survey of Bounder owners. With an average fuel economy of 7 miles per gallon the typical fuel requirement is 1714 gallons. A price increase of $1.00 per gallon results in an increase of $1,714 - certainly significant.

How can you reduce the cost? The obvious answer is drive less. Suppose you reduced your annual mileage to 10,000 miles. That would save $286.

Now let's look at all that we carry with us. Do you need everything? Can you travel with less water, tools or tinned goods etc.? We make a habit of going through our Bounder each year and looking at everything, we carry. You would not believe how many items we think we need to have with us, yet never use in a year. If it is unused, we remove it. When we travel, we are conscious of the space things that we acquire along the way take and their contribution to our overall weight. It is generally agreed that a typical part-time RVers carries about 2,000 lbs. of "stuff", while a full timer carries over 3,000 lbs. Your mileage will improve however the amount is difficult to estimate.

The most significant step you can make is to drive wiser.

  • Ensure that your tires are fully inflated to carry the load you are carrying.
  • Check your routing to select roads that avoid stop and go traffic.
  • Do not let your engine idle unnecessarily; it is far better to drive and warm up your engine under light load than it is to idle.
  • Apply steady pressure on the accelerator and do not pump or fluctuate your pedal pressure.
  • Anticipate your starts and stops and gradually increase speed,
  • Try to time stoplights to drive through on a green light. Since many traffic signal lights are computer controlled and sequenced, if you maintain a steady speed you should be able to approach all lights when they are green.
  • Use the overdrive gear on your transmission as long as road and wind conditions allow it.
  • Use your cruise control when possible and set a fuel efficient speed.
  • Reducing your road speed will save fuel. The enemy of motorhome fuel economy is drag and while we tend to focus on the frontal shape of our rigs; the reality is the rear section creates significant drag. Look at how much dust appears on the rear of your coach especially if you have one of the older recessed back models, to visibly see how the air travels around it. Drag horsepower is a function of Speed to the power of 2.86, slightly less than Speed cubed. The more horsepower you require, the more fuel you will consume. Reducing your speed from 65 to 55 MPH (a 15% decrease in speed) will reduce the drag horsepower by 39%. This could translate into about a 1 mile per gallon increase in economy. Combine the savings with your reduced travel distance and you will save $464.
  • The weight reductions could save another $50 or more.

To maximize your fuel economy, I recommend using a vacuum gauge in combination with the tachometer, on gas rigs. This gives a visual indication of engine efficiency. The higher the vacuum at any road speed, the better the fuel economy. Maintaining engine speed at the maximum torque rating of the engine in conjunction with use of a vacuum gauge will save as much fuel as possible. Reducing intake resistance and exhaust backpressure will also improve fuel economy provided that one does not use the improved throttle response to accelerate quicker and drive faster, offsetting the advantage of these improvements. In the next Tech Tip, I will talk about diesel engine fuel efficiency.


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