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2019+ BMW 3 AND 4-SERIES FORUMS (G2x Generation) General G20 Sedan / G22 Coupe / G26 Gran Coupe Discussions Which gear to use?

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      03-18-2020, 04:33 PM   #1
M40iSFR
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Which gear to use?

I've previously always driven six speed manual cars; 3rd gear was for around town, 4th dual carriageways, 5th the motorway. My new car has an automatic transmission with 8 forward gears to choose from - too many!

Say I'm sticking to the 120 KPH speed limit, 8th gear is most efficient, but gutless. I could easily use 7th, or 6th. I'm wondering what the real word difference in fuel economy is. Shouldn't be too difficult to calculate?
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      03-18-2020, 07:45 PM   #2
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Automatic set it and forget it
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      03-18-2020, 08:57 PM   #3
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Quote:
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Automatic set it and forget it
+1
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      03-18-2020, 09:07 PM   #4
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If you're "sticking" to 120 kph then why do you need a gear with guts? 8th is best. If you you want to overtake someone you can easily downshift, the transmission is very quick.
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      03-18-2020, 10:51 PM   #5
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Originally Posted by wtwo3 View Post
If you're "sticking" to 120 kph then why do you need a gear with guts? 8th is best. If you you want to overtake someone you can easily downshift, the transmission is very quick.
Or just step on the gas
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      03-18-2020, 11:14 PM   #6
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When you select drive, tilt the shift to the left which will engage sport mode and drop you down at least one gear. The BMW automatic is excellent and very fast. Stepping on the gas will get you into a lower gear in a hurry.
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      03-19-2020, 01:18 AM   #7
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+1

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      03-19-2020, 02:56 AM   #8
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What all others say. Plus, just look at your mpg in the screen to see the difference
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      03-20-2020, 08:21 AM   #9
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Automatic is no good for me. I like to be in control.

Does anyone actually know how to calculate the difference?
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      03-20-2020, 09:56 AM   #10
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Quote:
Originally Posted by M40iSFR View Post
Automatic is no good for me. I like to be in control.

Does anyone actually know how to calculate the difference?
So why did you get an automatic?

Your fuel economy will suffer, and your 0-60 (etc.) speed will suffer but if you want control, throw the gear shift to the left and simply use your paddle shifters (I assume you have them).

There is no real way to calculate the difference as your driving style will create wide variances.
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      03-20-2020, 04:09 PM   #11
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It must be possible to calculate.

At a constant speed of 120 KPH, gradient, rolling resistance, and all other variables constant. 8th gear would be say 1,000 RPM, 7th gear 1,500 RPM, etc. What's the difference in fuel consumption.

Maybe something to do with gear ratios?

Last edited by M40iSFR; 03-20-2020 at 04:15 PM..
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      03-20-2020, 04:48 PM   #12
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Quote:
Originally Posted by M40iSFR View Post
It must be possible to calculate.

At a constant speed of 120 KPH, gradient, rolling resistance, and all other variables constant. 8th gear would be say 1,000 RPM, 7th gear 1,500 RPM, etc. What's the difference in fuel consumption.

Maybe something to do with gear ratios?
A pointless exercise..... just enjoy the car....
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      03-20-2020, 04:59 PM   #13
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Thank you for your contribution.
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      03-20-2020, 05:00 PM   #14
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Quote:
Originally Posted by M40iSFR View Post
It must be possible to calculate.

At a constant speed of 120 KPH, gradient, rolling resistance, and all other variables constant. 8th gear would be say 1,000 RPM, 7th gear 1,500 RPM, etc. What's the difference in fuel consumption.

Maybe something to do with gear ratios?
One of the greatest logical fallacies is "all other things being equal" [They never are and on a public road, they can't be. So forget about it.
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      03-20-2020, 05:04 PM   #15
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Maybe someone who can help with the answer will come along...
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      03-20-2020, 06:22 PM   #16
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Quote:
Originally Posted by M40iSFR View Post
It must be possible to calculate.

At a constant speed of 120 KPH, gradient, rolling resistance, and all other variables constant. 8th gear would be say 1,000 RPM, 7th gear 1,500 RPM, etc. What's the difference in fuel consumption. ...
Yes, it is possible. Car is calculating it and display the results right in front of you? Am I missing something here?

Last edited by hamera; 03-20-2020 at 06:33 PM..
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      03-20-2020, 06:29 PM   #17
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Quote:
Originally Posted by themnmd View Post
Automatic set it and forget it
Yep.
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      03-20-2020, 06:40 PM   #18
M40iSFR
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Quote:
Originally Posted by hamera View Post
Yes, it is possible. Car is calculating it and display the results right in front of you? Am I missing something here?
Yes, the word 'constant'.
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      03-20-2020, 06:54 PM   #19
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Quote:
Originally Posted by M40iSFR View Post
Yes, the word 'constant'.
Yeah I get that. You do constant speed in constant conditions. You will get constant reading. Change gear and compare. No?
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      03-20-2020, 07:23 PM   #20
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You could never reproduce exactly the same conditions though. Even if you slammed on the brakes, reversed and drove the same section of road in a different. Tyre temp, engine temp, wind direction etc would all be factors.

It would have to be a theoretical calculation that keeps all other variables constant. If 6th gear ration was 2.3, 7th 2.1, 8th 1.9 you could calculate the increase in engine revolutions, and the relationship between fuel consumption. But I'm sure there are other factors too.

People don't need to contribute if the thread doesn't interest them. Not looking for opinions. I'm happy to leave it here until an engineering geek finds it.

Last edited by M40iSFR; 03-20-2020 at 07:29 PM..
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      03-20-2020, 07:53 PM   #21
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Quote:
Originally Posted by M40iSFR View Post
You could never reproduce exactly the same conditions though. Even if you slammed on the brakes, reversed and drove the same section of road in a different. Tyre temp, engine temp, wind direction etc would all be factors.

It would have to be a theoretical calculation that keeps all other variables constant. If 6th gear ration was 2.3, 7th 2.1, 8th 1.9 you could calculate the increase in engine revolutions, and the relationship between fuel consumption. But I'm sure there are other factors too.

People don't need to contribute if the thread doesn't interest them. Not looking for opinions. I'm happy to leave it here until an engineering geek finds it.
Theoretical calculations are just that...theoretical, and largely meaningless in the real world scenario you are dealing with. You seem a little fixated on theoretically calculating something, whereas a real-world and more meaningful way of assessing what you have asked has been suggested. But you dismiss it. I think you must be an astro physicist to be so obsessed with "theoretical" calculations!

By the way, I am an engineer.
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      03-20-2020, 08:04 PM   #22
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Maybe someone who can help with the answer will come along...
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