Most fuel efficent RPM
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Most fuel efficent RPM - 6/2/2008 5:41:31 PM
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thedfo
Posts: 82
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So, we all know that each engine is designed to have a sweet spot that has peak performance and should be the most efficent. However, we also all know that high RPM's mean lower MPG. How does one make these two statements work?
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Supporting the American Police Institutions one speeding ticket at a time.
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RE: Most fuel efficent RPM - 6/3/2008 10:14:12 AM
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davidteachey
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Well, peak fuel efficiency should be achieved by a combination of going as slowly as you can safely get away with, tires being properly inflated, a/c off, and use of cruise control. As you correctly mentioned, adding power detracts from this. The key, I think, is to travel at the slowest speed that's safe and practical. Air resistance KILLS efficiency disproportionally as speed increases. Use of the cruise control is probably the next biggest "helper" towards efficiency.
David
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David Teachey - '04 Mazda6 S Wagon V6 Automatic
Charlotte, NC, USA, Earth
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RE: Most fuel efficient RPM - 6/3/2008 4:26:25 PM
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virgin1
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You forgot windows up, but hit most of the other points... as near as I can figure.
I have to say here, as (one of) the resident smart-asses, that when I saw the title of this thread I was about to answer 0 rpm's affords the best mileage.
Are we getting just a little bit silly over fuel mileage these days?
I am old enough to have lived through both gas "crisis" in the '70's and it never fails to amuse me how when prices are considered normal, or "cheap," how many people go out and buy their big-assed Cadillacs, Lincoln's, pick-up trucks and SUV's. But when gas prices are up suddenly, every one seems worried about mileage... even those that weren't before.
Sales for bigger vehicles go way down and the car companies and dealers suddenly offer deep discount incentives to sell them. Those that own these gas guzzlers are selling and trading them in at an alarming rate and usually at a loss.
I on the other hand, drive smaller, sportier, fuel efficient vehicles because I like them and always have.
Even w/my heavy right foot and sporting driving style manage to achieve 25-26 w/A/C and 27-28 w/o in my 3. How much better do you think it's going to get than that, and at what cost?
BTW: I have always heard that an engine is at it's most efficient at the crossover point (torque/horsepower) on a dyno graph. That's when it's breathing the best, properly filling the cylinder w/a correct, well mix of fuel to air and burning that fuel most efficiently.
That does not afford you the best fuel mileage however, but the engine is running at it's most efficient at that point.
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 "There is no tool you can buy that will replace experience." - Josh Mills, C.K.DeLuxe
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RE: Most fuel efficient RPM - 6/4/2008 6:14:55 AM
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davidteachey
Posts: 170
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Actually, I didn't forget "windows up"...I chose not to include it. Per an SAE study, outlined in Road & Track magazine by their engineering editor Dennis Siminaitis, the drag that's created by open windows only starts to affect fuel economy once you're well into triple-digit speeds. Most of us are not cruising for any extended period of time at above-triple-digit speeds, so it's mostly a non-issue. They pointed out that, at normal highway speeds, the drag created by open windows is negligible at most. Whereas a/c, of course, taxes the engine directly. For me, all of this is gonna become little more than lip-service soon, because here in NC we're only about a month away from high-90s / high-humidity...my a/c will be on ROAR!!!
David
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David Teachey - '04 Mazda6 S Wagon V6 Automatic
Charlotte, NC, USA, Earth
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RE: Most fuel efficient RPM - 6/4/2008 7:21:10 AM
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virgin1
Posts: 4849
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Here in Texas it's been at or near 100* for the last week. My A/C is on ROAR even at 4AM!!
I am familiar w/Dennis Simanaitis (and his difficult-to-spell last name) and his articles. I was reading his work just this morning. Tech Tidbits is one of my favs in that mag... one of the first places I go w/a new issue. Been reading R&T for over 30 yrs now, but since we are getting "gas mileage silly" here....
Some of you may remember the VW Beetle TV ad back in the '70's. They managed 80mpg from a std Beetle, but there was so little left of it (they stripped every body panel and amenity possible) that it wasn't practical to drive.
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"There is no tool you can buy that will replace experience." - Josh Mills, C.K.DeLuxe
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RE: Most fuel efficient RPM - 6/4/2008 9:01:26 AM
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davidteachey
Posts: 170
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Hah, yes, he does have a tough last name! I was spelling it from memory, so sorry for the goof. I've been a R&T subscriber for about 26yrs, so you've got me beat by a little bit. My fave monthly article is Side Glances by the fabulous Peter Egan.
David
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David Teachey - '04 Mazda6 S Wagon V6 Automatic
Charlotte, NC, USA, Earth
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