Contradictory tips. - Fuelly Forums

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Old 08-24-2008, 07:16 AM   #1
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Contradictory tips.

There's a discrepancy between two tips on the tips page.

"Accelerate a bit...sometimes

Though some people will tell you never to accelerate quickly, sometimes it's better to have a bit of a lead foot. Seriously. There are two main reasons for this.

First: your engine likely operates at a higher efficiency (i.e. burns more fuel that would otherwise be wasted by your catalytic converter and just go out your exhaust) at around 80% acceleration.

Second: Your car gets better MPG at higher speeds up to about 55-65 mph depending on your car. So as long as there aren't too many red lights ahead of you, it's best to get your car to the maximum legal speed as soon as possible.
by jjmatt33 on August 08 2008"

and,

"Kill the Jackrabbit
Punching the gas from a stop (aka "jackrabbit starts") is a great way to waste fuel. You can optimize gas by starting from a stop slowly and gradually. Generally, drive as if you are taking a baby home from the hospital -- smooth, slow, and careful, to save the most fuel possible.
posted by mathowie on July 31 2008 "

Well which is it (or am I missing something)?
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Old 08-24-2008, 10:16 AM   #2
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I think it's totally ok to have seemingly contradictory advice in the tips section since there are many edge-case situations where the opposite of any common tip may be true. I keep an open mind when I'm approving them and generally go for a broad range of tips and might overlap or contradict each other, as long as the message of the tip is a good one.

In this specific case, I think we're talking about two different situations. The jackrabbit starts are referring to the time that you car leaves from a stoplight, and generally I'm saying going from 0mph-10mph by stomping on the gas is a horrible idea and it wastes a lot of gas (though leaving off the line very very slowly still gets poor but not as bad as jackrabbit start mileage for reasons covered in the second tip).

The second tip is referring to the behavior of internal combustion engines, which generally do poorly at fuel efficiency below about 40mph and generally peak around 50-55. There are times that if you are going 20mph and you're entering a freeway, it's fine to stomp on the gas and just get the 20-55mph acceleration over with quickly so you can get the car into a more efficient state and start saving fuel.

I got a ScanGauge the other day and have been monitoring my mpg closely in real time and I'm planning to write it up here in the Fuel Talk forum because it reveals the truth to a lot of things like that second tip.
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Old 08-24-2008, 11:32 AM   #3
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@mathowie

"I got a ScanGauge the other day and have been monitoring my mpg closely in real time and I'm planning to write it up here in the Fuel Talk forum because it reveals the truth to a lot of things like that second tip."

I'd like to see the results.
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Old 08-24-2008, 11:33 PM   #4
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I think one of the big issues with the fuel tips is that strategies vary quite a bit between cars. Let's say you drive an American sedan with a pushrod V6. In this situation, the engine is generally optimized for lower RPM, and you'll get better fuel economy keeping the revs below 2500-3000 while accelerating, since this rev range is where the engine produces power most efficiently. On an economy car with a DOHC 4cylinder, you'll do better by revving a bit higher while accelerating, since the most efficient operation on these engines is most like the 3000-5000 rpm range.

I do take serious issue with the one tip titled "Watch your RPMs." While I agree that high revs do reduce fuel economy by increasing engine friction, the posters begins by claiming "Despite common belief speed is not really connected to MPG." This is in fact completely untrue. Drag forces dominate at highway speeds. The drag force acting on your vehicle is equal to the drag coefficient * frontal area of the vehicle * square of the velocity of the vehicle. Therefore, every time you increase your velocity by ~1.4, you double the amount of power you're using to overcome drag, e.g. drag forces are twice as high at 79 mph as they are at 55 mph. These forces quickly overcome engine / drivetrain losses.
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Old 08-25-2008, 03:36 AM   #5
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well rem83, with my scangauge installed, I can see what the "Watch your RPM" tip refers to. There are specific low rpm points where my automatic transmission shifts up and if you can hold the speed and rpm down you can get a pretty significant mpg boost (on my truck going 44mph is enough to get into the top gear and on flat ground my ~18mpg truck reports 25-35mpg at that speed/rpm)
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Old 08-25-2008, 04:30 AM   #6
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I don't have a problem with the tip itself, the part I have the problem with is where the tipster completely discounts the effects of aerodynamic drag and claims that it's a common misconception.
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Old 08-27-2008, 01:55 PM   #7
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@ rem83

I assume you are talking about my tip "Watch you RPM's" as I don't see where the two quoted tips mention aerodynamics.

In any case, I was referring to velocity as a determining factor in MPG as a misconception. I never meant to imply aerodynamics did not have an effect.
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Old 08-29-2008, 02:09 PM   #8
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rem83 please read your quote carefully. It says exactly the opposite you seem to think it says. He's saying that a lot of people seem to think speed (and therefore drag) doesn't have an effect on MPG but he himself believes it does.
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Old 09-02-2008, 06:34 AM   #9
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@nakio30 - The way I interpreted the quote included a comma between the words belief and speed. If you interpret it to say that "speed is not really connected to MPG" is the common belief, then it is poorly worded and a sentence fragment (it would be better to say "despite the common belief that speed is not connected to mpg" which would still be a fragment), which seemed less likely to me than a missing comma, particularly given the emphasis of the tip on engine RPM and not total speed.

That and Tiki's previous comment both seem to suggest that your interpretation is actually incorrect and the sentence should read "Despite common belief, speed is not really connected to mpg."
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Old 09-04-2008, 03:31 AM   #10
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Also thought I'd throw this article from tirerack.com out there: http://www.tirerack.com/tires/tirete....jsp?techid=29

While it's certainly not perfect, the author's estimate is that at highway speeds aerodynamic losses account for about 60% of your vehicle's fuel consumption, while driveline friction (essentially engine RPM - of course, the article doesn't take into account efficiency changes over the engine's available RPM) is about 15%. During city driving, driveline friction accounts for ~45%
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