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Old 04-26-2008, 03:17 PM   #1
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Is driving slower also driving greener?

I was going 55 mph on a 2.5 hour trip yesterday and while doing that I was wondering if someone that did the trip doing 55 mph (and thus saving gas) compared to someone doing that same trip doing 65 mph would drive 'greener' or less green when it comes to total emissions for the whole trip.

Going slower sounds greener, but maybe the engine burns more efficiently at higher speeds and thus the total of emissions for the whole trip would be less?In other words, do saving gas and saving the environemnt go hand in hand or not?
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Old 04-26-2008, 03:34 PM   #2
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I'd think slower is better within the same gear. Speed is still controlled by the amount of fuel that is injected. I don't see how using more fuel for the same distance could add up to less emissions.
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Old 04-26-2008, 03:47 PM   #3
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My 94 Del Sol:
75-80=36MPG
65-70=39MPG
55-60=44MPG

At 70 MPH aerodynamic drag is about 70% of energy required. It increases as the square of speed.

Engine peak efficiency is about 70% of wide open throttle at 1200-2400 RPM (fuel consumed per horsepower produced).

This is the reason pulse and glide is more efficient.

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Old 04-26-2008, 11:41 PM   #4
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id say slower, sure your going slower but like others have said your burning less and since its not going thru the 4 strokes as fast(might not make a difference between ohh 2K and 2.5K tho) it might burn the fuel air mix more efficiently...
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Old 04-27-2008, 05:24 PM   #5
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slower... to a point

except my truck which seems to increase with speed. my best mpg coincided with the fastest constant speed. 27-28 mpg@ 65-70 mph, 29 mpg @ 75 mpg. I'd test more but I've altered the aerodynamics of the truck (running an e-fan and no mech. fan so I unblocked the radiator to get natural airflow and not run the efan 100% duty cycle.)
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Old 04-27-2008, 06:02 PM   #6
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I think the slowest I can drive in Marvin for best FE is 70kph/43mph, that's where I can get the TC locked up, I can ease it back just a little and it will stay in as long as the road is dead flat, there's no wind and I don't have to turn the wheel, but that barely ever happens so it's best just to cruise there. Theoretically should get about 35mpg at that speed.
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Old 04-27-2008, 06:10 PM   #7
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Quote:
Originally Posted by kamesama980 View Post
except my truck which seems to increase with speed. my best mpg coincided with the fastest constant speed. 27-28 mpg@ 65-70 mph, 29 mpg @ 75 mpg. I'd test more but I've altered the aerodynamics of the truck (running an e-fan and no mech. fan so I unblocked the radiator to get natural airflow and not run the efan 100% duty cycle.)
I'd figure maybe it takes a certain speed before a vortex cushion forms behind the cab, making a virtual kammback.
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Old 04-27-2008, 09:04 PM   #8
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You need to separate the time and distance factors when talking about this.

Taking a simple example...
A trip of 35 miles uses 1 gallon so the F/E is 35 MPG.

Car A takes one hour and car B take twenty minutes but if both cover the same 35 miles using 1 gallon each then both are equal in the F/E status and , since both have burned the same volume of fuel , the emissions would also be the same or virtually so.
I am assuming here both vehicles have the same technology to manage emissions levels.

The time taken is not a factor in the consideration of fuel used to cover a given distance.
Engine capacity is in the same bracket which is a point many people also miss.

It can be a consideration for the driver and passengers but that is another topic altogether.

Pete
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Old 04-28-2008, 03:27 AM   #9
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Originally Posted by Hateful View Post
I'd think slower is better within the same gear. Speed is still controlled by the amount of fuel that is injected. I don't see how using more fuel for the same distance could add up to less emissions.

It depends on what you are counting as emissions. If you are burning less fuel, then you are producing less carbon dioxide. But when it comes to other emissions, things get a little more tricky. This basically has to do with the efficiency of the cat. In theory, running at a lower RPM is not going to keep the cat as hot, which could mean more emissions of CO, HC, and NOx when driving slow rather than fast. Of course, it would be rather hard to check this while actually moving.
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Old 04-29-2008, 08:04 AM   #10
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I thought that the conclusive answer to this question is that each vehicle has a sweet spot, and it differs from one to the next...is that not true?
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