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Old 11-18-2007, 07:48 PM   #1
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Save fuel without slowing down

Save Fuel Without Slowing Down

Hi, folks,

I thought this should be of interest to everybody. Most of us know you can save a lot of fuel by slowing down, maybe as much as 10% by cutting speed 5 mph. How you drive matters a lot also. Let's look at the energy numbers associated with driving 60 mph in a 3800 lb car with a couple of passengers-- say, a total weight of 4500 lb including a tankful of fuel. We could expect a car like that to get about 24 miles per gallon at a steady 60 miles per hour. (That's a little higher speed on the average speedometer.)

If you work out the power needed at the wheels for this 24 mpg, it is 14.6 kilowatts, or 0.243 kWh /mile. (Using typical parameters for a U.S. car.)

Now, let's figure out how much kinetic energy the car is carrying at 60 mph. (60 mph = 26.82 meters /sec.)

KE = 1/2 M V^2 = 0.5 x (4500 /2.2) x (26.82)^2 = 735,660 joules = 0.204 kWh.

If the car is brought to a stop quickly, then that 0.204 kWh is wasted making hot air.

Well, then, if the car makes one panic stop every mile, the energy consumption per mile is
0.243 + 0.204 = 0.447 kWh, which means the car will be getting only 12.9 mpg.

But, if you can coast to a stop without using the brakes, then the kinetic energy of the car is translated into forward motion and used in place of fuel. Let's say that by coasting to stops it could be practical to effectively eliminate the energy loss of one sudden stop every two miles. In that case, the car will be getting 16.8 mpg. That is a 30% gain in fuel economy, compared to 12.9 mpg, by just going light on the brake pedal without otherwise changing speed.

Is this realistic? Yes, I think so. I have personally measured differences of about that much while driving my Beetle. And, 30% variation in MPG among drivers is just the number quoted in the Cummins MPG Guide, on page 25.

Ernie Rogers

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Old 11-18-2007, 08:21 PM   #2
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That is interesting. Problem is, with so many cars on the road, coasting to a stop is often not practical for the drivers behind me. But I will attempt not stop so hard in light of this. Thanks.


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Old 11-18-2007, 09:21 PM   #3
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Drive slower + coast

Driving slower also gives you more time to observe your surroundings to anticipate changes in conditions such as cars turning onto the road - or off.

In any case, KE is conserved... So exactly the same thing applies at any speed. But the faster you go, the more energy is transfered into the air (drag) rather than continuing forward motion

^^ of course, all of that is conditional. If you have more open spaces and longer distances between traffic events - you can afford to coast down from higher speeds.
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Old 11-18-2007, 11:32 PM   #4
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Ernie: What you have just described is one of the pillar stones of hypermiling. It is nice to see it broken down with mathematical formulas, but most people who are serious about this already employ that technique.

The idea of hybrids is that they help people overcome the breaking losses that cars present, when not driven in the above explained way. That is why we can see people like COZX2 getting mpg in the 100s driving a normal everyday car. Most cars if driven properly are able to get phenomenal mileage. Just depends on the person behind the wheel, and where said car is driven.

For anyone out there that doesn't use "driving without brakes," I highly suggest you adopt the technique as soon as possible. You get used to the squealing sounds going around curves, trust me.
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Old 11-19-2007, 12:11 AM   #5
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Ernie -

Thanks for pointing out the PDF. It has very good info. I like this graph :

Attachment 1091

I also like this one :

Attachment 1092

Even though the graph applies specifically to boxy non-aero trucks, it gives you a great idea how more HP is needed to keep a vehicle moving at high speeds. Maybe the curves would show different proportions for passenger cars (or not?!?!?!?), but the overall idea is the same.

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Old School SW2 EPA ... New School Civic EPA :

What's your EPA MPG?
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