Saving Gasoline through Smarter Driving

Last Up date: 2006 August 26
Recent changes: Link to my thoughts about using a Cell phone while driving.
Page started: 2006 August 24

Since starting this page I have found more data at: Rcumbents.com I assume from the name they are into Recumbent bicycles, at the bottom of their page there is a link to: "Vehicle Mileage as a function of speed" which has some interesting data.

I have known for some time that "air drag" goes up as the square of the velocity, when traveling at speeds in the range of about 20 to 100 MPH. (At walking speeds it is approximately linear, and at jet plane speeds it is more like the cube of the velocity.) I don't know how friction drag in such things as the engine, transmission, and tire flexing, varies with speed? Incidentally the energy that has to be dissipated in an accident also goes up as the square of the speed. Here is a page about saving lives by lowering speed limits. And, in the interest of safety, I have thoughts about using a Cellular Phone while driving and laws that could reduce that hazard.

A friend has been experimenting with reducing his driving speed to see the effect on the gas savings. It is quite impressive. If I can get him to summarize in an email, I think it would make an interesting page.

Here are his notes:

These data are all for a 2002 Toyota Camry with the 2.4 liter 4-cylinder engine and 4-speed automatic transmission. The EPA mpg numbers for this car were 23 city and 32 highway.

The numbers from four different driving scenarios:
  1. San Francisco Bay Area to Tucson, AZ and return in 2004 driving with traffic (an estimated average of 4 to 5 mph over most posted speed limits). MPG = 32.9
  2. San Francisco Bay Area to Tucson, AZ and return in 2006 driving 4 to 5 mph under posted speed limits or about 8 to 10 mph slower than in case 1 above. MPG = 35.6
  3. San Francisco Bay Area to Boise, Idaho in 2006 driving 55 mph on 2-lane rural highways and 90 percent of the speed limit on interstate highways (e.g., 63 mph where the posted speed limit was 70 mph) MPG = 39.0
  4. Boise, Idaho to San Francisco Bay Area in 2006 driving 50 mph on 2-lane rural highways and 55 mph (the California speed limit for vehicles, including big rigs, towing trailers) on interstate highways MPG = 42.9

So, rounded to the nearest mile, those numbers are 33, 36, 39 and 43! Reducing velocity has a very significant effect! From experience, the formula in scenario 4 is about a slow as one can go without becoming a serious obstruction and creating a risk to ones own safety and does require careful observation of the situation developing in ones rear view mirror!

All four scenarios were not exclusively highway driving but included a small but significant amount of city driving. The two Arizona trips were in March and included very little use of air conditioning (cooling). The Idaho trips were in August and included use of cooling perhaps 10 to 15 percent of the time. Driving slower than the prevailing traffic flow is hard to do, almost an "unnatural act" for many Americans! But, it can be done. Just set the cruise control and keep your foot away from the pedal - no "pedal to the metal."

For comparison, the EPA numbers for the 2002 Toyota Camry with the 3.0 liter 6-cylinder engine were 20 city and 28 highway, a 14 percent reduction in EPA highway mpg.

Here's some more data.
Credit the February, 2006 issue of Car and Driver magazine.

They tested a 2006 Lexus RX400h Hybrid SUV (similar to the Toyota Highlander Hybrid SUV). Both vehicles have EPA numbers of 31 mpg City and 27 mpg Highway for the All Wheel Drive version (33 and 28 for the Front Wheel Drive version). This vehicle uses a 3.3 liter 6-cylinder engine with a Continuously Variable Transmission. The coefficient of drag (CD) is 0.35.

Here are the "steady state" mpg numbers Car and Driver got running at five different speeds both directions on a 13.7 mile course.

80 mph = 26.1 mpg 
70         = 31.7 
60         = 37.2 
50         = 41.0 
40         = 49.3

These numbers certainly support the "Stuart Method!"

Additional information: The coefficient of drag (CD) for the 2002 Toyota Camry is 0.28.

Also, you may find it interesting to look at this Wikipedia article on Coefficient of Drag Note the CD for the Cessna 172/182! Of course, total drag is a function of many things such as: tires and their pressure, wind direction and speed which effectively changes your air speed, the frontal area, and the CD.

The numbers given above would have even been higher without Air Conditioning, but how much I am not sure. I have heard people claim rolling the windows up reduces the air drag enough to more than compensate for the power used by the air conditioner. I frankly don't believe this.

Cooling an automobile is a significant air conditioning load compared to cooling a room in a house because of all the windows, metal body which conducts heat much better than wood, brick, or stucco thin insulation, and leaks of the air going around the moving vehicle; and people demand the cooling almost instantly in a car, where they are quite willing to wait hours for a home air conditioner--and may leave it on continuously. Typically window air conditioners, to cool a single room run from about 3 to 6 horsepower. I would guess that an automobile air conditioner takes 15 to 20 horsepower. If someone has data on this it would be interesting.

Certainly, at very low speeds the rolling down of a car window produces very little drag but the air conditioner load decreases very little at low speeds or even stopped. It only takes about 15 horsepower to move a Toyota Prius at 60 MPH on level ground.

Rolling down a window is not going to double the load. At jet aircraft speeds rolling down a window may increase the drag enough to make it more efficient to use an air conditioner. But, at normal speeds and certainly in City driving you can save gasoline by turning off the AC. Researching this I discovered a great scientific web page by a Physicist, at Univ. of Chicago. He has the following statement:

A friend recently pointed out to me that the effect of the electrical load of the headlights is probably far greater (by percentage) for hybrid cars! He mentioned a fact that most people are probably unaware of, that when the A/C is running, hybrids cannot get over around 30 mpg! Since most people use their A/C most of the time, that pretty much defeats the whole purpose of a hybrid having the capability of having higher gas mileage. He said that a lot of people who buy hybrids soon sell them again when they discover the much lower mileage they were getting because they used their A/C.
The above is from his great page about the cost of driving with the headlights on. For his entire page click HERE.

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