# Most efficient cruising speed?



## avalys (Oct 14, 2003)

I have a 2003 325Ci with a 5 speed manual transmission, and while driving to work the other day a question occurred to me. If I wanted to get somewhere using the least amount of gas possible, how fast should I go? At some point, the increased efficiency of the engine at high speeds must begin to lose out to the increased drag from air resistance, but what is the exact speed at which this occurs?


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## Alex Baumann (Dec 19, 2001)

56 mph (which is 90 km/h)

EDIT : That being said, important is to drive at constant speeds.


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## Alex Baumann (Dec 19, 2001)

rumratt said:


> Where'd you get that number?


It's one of the constant numbers that car magazines test the consumption figures. The other one is 120 km/h (74 mph)


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## Cliff (Apr 19, 2002)

Alex Baumann said:


> It's one of the constant numbers that car magazines test the consumption figures. The other one is 120 km/h (74 mph)


 How do they feel about 150 km/h (93 mph)


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## Alex Baumann (Dec 19, 2001)

Cliff3 said:


> How do they feel about 150 km/h (93 mph)


The driver happier, the wallet feels itself like s-hit though


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## Miles_1 (Jul 26, 2004)

I recently read that (speed be damned, it's all about RMP's) thermodynamically speaking, for all engines, a gasoline fueled engine is most energy efficient at 2000 RPM's. So, maybe, it depends upon one's definition of "efficient."


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## Nick325xiT 5spd (Dec 24, 2001)

Miles said:


> I recently read that (speed be damned, it's all about RMP's) thermodynamically speaking, for all engines, a gasoline fueled engine is most energy efficient at 2000 RPM's. So, maybe, it depends upon one's definition of "efficient."


 Maybe so, but there are other factors. I'll bet that doesn't factor in mass, drivetrain loss, and drag.


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## jgrgnt (Apr 27, 2002)

Maybe someone can take OBC readings at different speeds.


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## shizat63 (Feb 14, 2002)

Don't know how accurate the MPG guage is, but one day on a perfectly flat strech of road I can get ~28mpg at 80mph. But another day I can get over 35mpg. Needless to say I wasn't accelerating in either scenario.

If you lift up on the gas pedal ever so slightly, the MPG guage will go up but the speed will not change.


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## TLudwig (Mar 30, 2004)

A big myth is that if you set your cruise control on a constant speed, you will have the best gas mileage possible. That is only true if you are on a perfectly flat stretch of highway. Normally, however, you have rises and drops, either in the form of hills or highway overpasses. It is best to let your speed drop a little bit as you go up the rise and accelerate back up to your cruising speed as you go down the other side. This can make a big difference even on a flat road with a number of overpasses.


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## AndDown (Jun 17, 2003)

shizat63 said:


> Don't know how accurate the MPG guage is, but one day on a perfectly flat strech of road I can get ~28mpg at 80mph. But another day I can get over 35mpg. Needless to say I wasn't accelerating in either scenario.
> 
> If you lift up on the gas pedal ever so slightly, the MPG guage will go up but the speed will not change.


Finally...someone who uses the MPG gauge! :thumbup:


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## atlau (Jul 1, 2004)

I made a query about that recently... other recommendations include:-

*Aircon - leaving it off
*Lights - leave it off
*Radio - leave it off
*Spare tire - remove it
And any other options u think are deemed unnecessary and hence can remove (the chairs? Airbags? ABS?)


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## Jim H (Feb 9, 2004)

TLudwig said:


> A big myth is that if you set your cruise control on a constant speed, you will have the best gas mileage possible. That is only true if you are on a perfectly flat stretch of highway. Normally, however, you have rises and drops, either in the form of hills or highway overpasses. It is best to let your speed drop a little bit as you go up the rise and accelerate back up to your cruising speed as you go down the other side. This can make a big difference even on a flat road with a number of overpasses.


Yep, agreed. I read an article in Car and Driver a bunch of years ago about how they drove a Honda Civic (it was the model that at the time had Honda's best mpg rating) from LA to San Fran, trying to get the best mileage they could get. By slowing down up hills, and speeding up on the down side, they got almost 65 mpg. Of course, that was a car that easily got over 40 mpg on a regular basis.


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## SergioK (Aug 14, 2002)

Time is sometimes money... so at _time_s, it may be more efficient for one's pocketbook to decrease the amount of time needed to reach a destination in lieu of worse MPG effeciency.


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## grayghost (Jan 10, 2003)

AndDown said:


> Finally...someone who uses the MPG gauge! :thumbup:


Trying to use the "MPG" guage as a true and accurate indicator of miles/gallon is unreliable at best.... That is really a "Manifold Vacumn Guage" (Or, "Mainfold Pressure", if you prefer.) There are so many variations in how it will read under various circumstances, that tryng to use it as an accurate indicator is hopeless. About all you can say is that, in the same gear and at the same approximate speed, it will indicate better or worse mileage. It is very easy to indicate a better MPG in a lower (4th) gear than in a higher (6th) gear in otherwise equivalent conditions. 
For what it's worth, by being careful and cruising between 70 - 75 MPH, I got slightly over 32 MPG on a trip this weekend. And that is with a little (10%) local/surburban driving. I found my 330Ci (6 sp manual) got right at 32 MPG cruising at 75 (true speed, not speedometer indicated). I figure that is not at all bad, considering the performance that is possible if you are not trying for good MPG.


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## Fast Bob (Jun 4, 2004)

You guys REALLY did`nt buy all those ZHPs hoping to get 40 mpg, now did you????
(Three grand in sixth is pretty good....)

Regards,
Bob


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## HW (Dec 24, 2001)

Alex Baumann said:


> 56 mph (which is 90 km/h)


i've heard that from my driving instructor a long time ago. supposedly cars are designed to be most efficient at that speed. :dunno:


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## ctbmw (Aug 30, 2003)

I use cruise control as often as possible. If I don't I'm flying during a fast song, or loafing during a slow one.
The only time I click it off if not traffic related is if I'm going down a hill, car will then pick up speed a little more.
Are highway speeds going down where you live? It seems like people are driving a little slower lately-maybe it's just me.


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## Kaz (Dec 21, 2001)

SAAB, I think did a study on this a number of years ago. They took a car (a 900?) to an evenly banked track and ran the car nonstop at some set speed. I want to say it was 70 or 75kph.


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## TLudwig (Mar 30, 2004)

ctbmw said:


> I use cruise control as often as possible. If I don't I'm flying during a fast song, or loafing during a slow one.
> The only time I click it off if not traffic related is if I'm going down a hill, car will then pick up speed a little more.
> Are highway speeds going down where you live? It seems like people are driving a little slower lately-maybe it's just me.


Yeah, it's hard to keep a consistent speed for any length of time on the highway, which is why I sometimes use mine. I can relate to the speed varying depending on the song. :rofl:

I know that Houston highways have gotten a lot slower. People use to fly on the highways (except during rush hour traffic). 7 years ago, the speed limit was 65 in most areas, and many people would do 10-15 mph over that. Several years ago, the city lowered the limit to 55 in most places and did a big enforcement campaign, so now most people drive much slower than they used to. OF course, now that I have a BMW, it seems like almost everyone drives slow now.


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