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Old 11-22-2006, 08:10 AM   #1
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Speed comes at a price

Speed comes at a price....

65 mph vs 60mph = 6.4% decrease in fuel economy
70 mph vs 60mph = 10.9% decrease in fuel economy
75 mph vs 60mph = 17.3% decrease in fuel economy


Pushing air isn't easy

Between 55 and 60 MPH, 50% of the fuel is used to overcome air resistance, the remaining 50% is used to overcome the load. The percentage of fuel needed to overcome air resistance increases dramatically as speed increases.


According to GM:

Drive the speed limit. Gas mileage decreases rapidly at speeds. On a typical SUV, for every 10 miles-per-hour average speed increase, you can lose four miles-per-gallon of fuel economy. In addition, for some drivers, using cruise control on the highway could help save as much as 10 percent.
http://www.gm.com/company/gmability/...ab_111805.html

There is a direct relationship between how much gas your car consumes and how fast you drive. It is a curve and the peak is around 55 mph. When you are going too fast or too slow, you use more gas.
http://auto.howstuffworks.com/framed...ning/speed.htm



As a rule of thumb, you can assume that each 5 mph you drive over 60 mph is like paying an additional $0.20 per gallon for gas.
http://www.fueleconomy.gov/feg/driveHabits.shtml

Conclusion.

Driving the speed limit, 7-23% increase in FE, saving $0.16-$0.53 per gallon
Reducing weight by 100 lbs, 2% increase in FE, saving $0.02-$0.05 per gallon

Note: Cost savings are based on an assumed fuel price of $2.31/gallon.


Annual FE improvement and money saved:


36 MPG car driving 400 miles per week
Cost per year = $1334

Same car with weight reduction and Reduced speed
25% total increase in FE.
45mpg and $1067 per year


You save: $267.00 per year
or $5.13 per week
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Old 11-22-2006, 10:08 AM   #2
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We have a common rule with TDI's called the "120 rule". Basically you take 120 and minus your speed and that’s your mpg. Of course this really doesn't work at speeds below about 30 mph and above about 80. But right about 60 it is darn close to 1 mpg for every mph. Even on my normal commute I see this happen when I drive 38 vs. 40 on scangauge. I get about 3 mpg better.
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Old 11-22-2006, 10:45 AM   #3
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120 rule, that is interesting

i wish things worked that way for me! I would be getting 65mpg at 55mph, but obviously that is not happening yet...
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Old 11-22-2006, 02:36 PM   #4
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Not always, while wind resistance is a factor, gearing is a big one. Look at the fuel ratings for large European cars of the 60s with and without overdrive, and the FE of them. Fun reading.
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Old 11-22-2006, 02:57 PM   #5
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Quote:
Originally Posted by FormulaTwo
Speed comes at a price....

65 mph vs 60mph = 6.4% decrease in fuel economy
70 mph vs 60mph = 10.9% decrease in fuel economy
75 mph vs 60mph = 17.3% decrease in fuel economy


Pushing air isn't easy
What aerodynamics are assumed in these numbers? I would think that the more aerodynamic the car design, the less FE would suffer at higher speeds (and conversely, the less aerodynamic the design, the more FE should suffer with increased speed).

After all, the whole issue here, is the FE loss of pushing air at faster and faster speeds. But at the same time, aerodynamic improvements also give the greatest FE gains at faster speeds (because they lower the overall coefficient of drag of the vehicle). So while the loss in FE from going faster might still be more than the added aerodynamic gains (making it still better, FE wise, to go slower), the difference speed makes in FE should be much lower for highly aerodynamic designs...
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Old 11-22-2006, 03:53 PM   #6
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Quote:
Originally Posted by theclencher
Aero isn't the whole issue; rolling and other mechanical resistances increase as rpms increase. But aero is the biggy.
Point taken.

However, the real issue here, is that those FE numbers are only realistic when taken in context (i.e. with info about how much cd is assumed in the vehicle). Otherwise, they fit the old cliche: "There are three types of lies: Lies; Damn lies, and Statistics..."

BTW: Thank you FormulaTwo for posting this. I appreciate the info. However, I just want to know the context of the numbers, so we can properly apply them to our situations (including those of us with cars more aerodynamic than the average USA fleet)...
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Old 11-25-2006, 05:04 PM   #7
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Cool

While true that there is more than exponentually more air resistance as you increase speed. This is a complete generalization, and not indicative of any vehicle in particular.


Calculating the approximate horsepower loss to aerdynamic drag is very easy. You simply need accurate frontal area, and the frontal cd to do so.
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