BMW X3 Forum
BMW X3 Forum
Welcome to the ultimate BMW X3 community.
BMW Garage BMW Meets Register Search Today's Posts Mark Forums Read
Post Reply
 
Thread Tools Search this Thread
      02-12-2012, 06:33 PM   #1
corey2140
Enlisted Member
8
Rep
41
Posts

Drives: 19' Hockenheim M2C DCT
Join Date: Jun 2011
Location: Chicago

iTrader: (0)

Highway fuel economy

For the 35i, at what highway speed does the car get the best MPG/ fuel economy? I would like to set my cruise control to that speed. And yes I'm on the highway right now
Appreciate 0
      02-12-2012, 08:07 PM   #2
paul386
Second Lieutenant
18
Rep
274
Posts

Drives: 2011 X3 35i Jet Black
Join Date: Oct 2009
Location: Florida

iTrader: (0)

The lower the speed the better. The car probably gets its best gas mileage between 30-45MPH. The energy required as a function of speed is a v^3 function. Double your speed and the energy required increased 8 times.
Appreciate 0
      02-12-2012, 08:44 PM   #3
Want the thrill
Lieutenant Colonel
326
Rep
1,560
Posts

Drives: 2022 X4M40i, retired ‘11 X3
Join Date: Mar 2011
Location: MI

iTrader: (2)

Garage List
2022 X4M40i  [10.00]
Going on cruise set at 75 mph for 4 hours I got 28 miles to the gallon. I was happy with that.
Appreciate 0
      02-13-2012, 08:32 AM   #4
renderfarmer
Enlisted Member
United_States
3
Rep
42
Posts

Drives: 2012 X3 xDrive35i M
Join Date: Dec 2011
Location: New Jersey

iTrader: (0)

Quote:
Originally Posted by paul386 View Post
The lower the speed the better. The car probably gets its best gas mileage between 30-45MPH. The energy required as a function of speed is a v^3 function. Double your speed and the energy required increased 8 times.
Theoretically yes, yet empirically I've now driven my 60 mile commute (highway) at about ever conceivable speed and i consistently get over 27mpg when i can maintain a steady speed of 65-70mph.

I think that modern engines and fuel management systems are too pro-active in maximizing performance to reduce fuel economy to a two term equation.

I just drove to and from Montreal (336 miles each way) and with 4 people on board and some light luggage I got 23mpg with an average speed of 70mph. The temp was -12C (10F) which didn't help matters.
Appreciate 0
      02-13-2012, 10:13 AM   #5
RhoXS
Second Lieutenant
15
Rep
233
Posts

Drives: 2002 Z3 3.0i + 2016 X3 28i
Join Date: Jan 2012
Location: Florida

iTrader: (0)

Quote:
Originally Posted by renderfarmer View Post
Theoretically yes ...
... The temp was -12C (10F) which didn't help matters.
IMO the theory is indeed reality. The losses that determine the actual fuel economy vary primarily with speed and driving practices.

Some losses are just a function of speed such as wind resistance. Since wind resistance is the really big hitter at constant cruise controlled higher "highway" speeds, the energy needed to propel the vehicle is indeed a function of the cube of the speed. At "highway" speeds there is essentially no braking or acceleration so windage is the only consequential energy loss.

At lower speeds" windage is negligible. The big hit energy losses are due to breaking where all the vehicles kinetic energy is converted to heat. This energy is recoverable (regenerative breaking) and that is why hybrids do so well in traffic but no better than non-hybrids at higher speeds where brakes are not used very much.

Of course, the weight of the drivers foot is also a factor as the vehicle's engine must abide by Newton's second law. That is, the force needed to accelerate a car (mass) is directly proportional to how quickly you want to accelerate it. Higher acceleration means greater force which means more gasoline. At a constant cruise controlled speed the ONLY energy needed is that necessary to overcome losses as the vehicles inertia (Newton's first law) will keep the car going forever if it was not for the losses.

The actual biggest energy loss is due to the inefficiency of the thermodynamic cycle that governs the engine. Greater than 2/3 the energy extracted from the gasoline is peed away out of the radiator and engine block. However, there is some point that the engine is most efficient so, when considering the rpm for optimum engine efficiency, braking, acceleration, and windage, that is why there is some speed at which the curve peaks. This point is invariably well below "highway" speeds. Bearing losses exist but are negligible so are typically ignored.

In other words, you can't F%$$ with mother nature when it comes to mpg. The faster you make the car go, the lower your mpg (unless, of course, you choose to drive in a vacuum). At highway speeds the aerodynamic slipperiness of the car is the dominant factor. At lower speeds the energy losses due to windage are essentially zip and braking/acceleration losses/inefficiencies dominate. A hybrid recaptures the low speed breaking losses but can't do squat for windage. Acceleration in a hybrid, is the result of a much more efficient energy conversion process (battery and electric motor vs an internal combustion engine).

BTW, temperature is a double edged sword. At lower temperatures the air is more dense. This means higher windage but more oxygen per unit volume of air sucked into the engine. I have no clue how significant these factors are or which dominates. I do remember from many years ago a friends GTO seemed to accelerate better on cold days.

Last edited by RhoXS; 02-13-2012 at 11:36 AM..
Appreciate 0
      02-13-2012, 10:36 AM   #6
x3heh
Private
United_States
1
Rep
79
Posts

Drives: Honda
Join Date: Jan 2012
Location: United States

iTrader: (0)

The less acceleration the better for fuel economy. Cruise control helps a lot.
Appreciate 0
      02-13-2012, 10:46 AM   #7
renderfarmer
Enlisted Member
United_States
3
Rep
42
Posts

Drives: 2012 X3 xDrive35i M
Join Date: Dec 2011
Location: New Jersey

iTrader: (0)

Quote:
Originally Posted by RhoXS View Post
BTW, temperature is a double edged sword. At lower temperatures the air is more dense. This means higher windage but more oxygen per unit volume of air sucked into the engine. I have no clue how significant these factors are or which dominates. I do remember from many years ago a friends GTO seemed to accelerate better on cold days.
It might improve acceleration but don't forget that the pressure in your tires changes 1psi for every 10F. Over the course of my trip to Montreal I saw a 30F swing in temperature.
Appreciate 0
      02-13-2012, 03:17 PM   #8
paul386
Second Lieutenant
18
Rep
274
Posts

Drives: 2011 X3 35i Jet Black
Join Date: Oct 2009
Location: Florida

iTrader: (0)

Quote:
Originally Posted by renderfarmer View Post
Theoretically yes, yet empirically I've now driven my 60 mile commute (highway) at about ever conceivable speed and i consistently get over 27mpg when i can maintain a steady speed of 65-70mph.

I think that modern engines and fuel management systems are too pro-active in maximizing performance to reduce fuel economy to a two term equation.

I just drove to and from Montreal (336 miles each way) and with 4 people on board and some light luggage I got 23mpg with an average speed of 70mph. The temp was -12C (10F) which didn't help matters.
I'm not sure your point? Try driving the same trip at 55MPH. You'll get better than 27mpg. Drive it at 40MPH. You'll get close to your best fuel economy there.

Quote:
Originally Posted by RhoXS View Post
IMO the theory is indeed reality. The losses that determine the actual fuel economy vary primarily with speed and driving practices.

Some losses are just a function of speed such as wind resistance. Since wind resistance is the really big hitter at constant cruise controlled higher "highway" speeds, the energy needed to propel the vehicle is indeed a function of the cube of the speed. At "highway" speeds there is essentially no braking or acceleration so windage is the only consequential energy loss.

At lower speeds" windage is negligible. The big hit energy losses are due to breaking where all the vehicles kinetic energy is converted to heat. This energy is recoverable (regenerative breaking) and that is why hybrids do so well in traffic but no better than non-hybrids at higher speeds where brakes are not used very much.

Of course, the weight of the drivers foot is also a factor as the vehicle's engine must abide by Newton's second law. That is, the force needed to accelerate a car (mass) is directly proportional to how quickly you want to accelerate it. Higher acceleration means greater force which means more gasoline. At a constant cruise controlled speed the ONLY energy needed is that necessary to overcome losses as the vehicles inertia (Newton's first law) will keep the car going forever if it was not for the losses.

The actual biggest energy loss is due to the inefficiency of the thermodynamic cycle that governs the engine. Greater than 2/3 the energy extracted from the gasoline is peed away out of the radiator and engine block. However, there is some point that the engine is most efficient so, when considering the rpm for optimum engine efficiency, braking, acceleration, and windage, that is why there is some speed at which the curve peaks. This point is invariably well below "highway" speeds. Bearing losses exist but are negligible so are typically ignored.

In other words, you can't F%$$ with mother nature when it comes to mpg. The faster you make the car go, the lower your mpg (unless, of course, you choose to drive in a vacuum). At highway speeds the aerodynamic slipperiness of the car is the dominant factor. At lower speeds the energy losses due to windage are essentially zip and braking/acceleration losses/inefficiencies dominate. A hybrid recaptures the low speed breaking losses but can't do squat for windage. Acceleration in a hybrid, is the result of a much more efficient energy conversion process (battery and electric motor vs an internal combustion engine).

BTW, temperature is a double edged sword. At lower temperatures the air is more dense. This means higher windage but more oxygen per unit volume of air sucked into the engine. I have no clue how significant these factors are or which dominates. I do remember from many years ago a friends GTO seemed to accelerate better on cold days.
I'd like to add a few comments to this.

At low speed losses are (in approximate order of significance)
- Heat lost in the engine
- Braking / Accelerating
- Vehicle accessories (A/C, steering pump, transmission pump, radio, lights, ect)
- Tire rolling resistance
- Aerodynamics

At high speeds losses are (in approximate order of significance)
- Heat lost in the engine
- Aerodynamics
- Tire rolling resistance
- Vehicle accessories (A/C, steering pump, transmission pump, radio, lights, ect)

Here is an interesting article Tesla Motors put together for their electric vehicles range versus speed. Relevant, but not perfect since it does not have a combustion engine!

http://www.teslamotors.com/blog/road...ency-and-range
Appreciate 0
      02-13-2012, 09:23 PM   #9
Nahoa
Lieutenant Colonel
United_States
45
Rep
1,876
Posts

Drives: F25 35i Mineral Silver/Black
Join Date: May 2008
Location: Virginia

iTrader: (0)

Quote:
Originally Posted by x3heh View Post
The less acceleration the better for fuel economy. Cruise control helps a lot.
Not so. Keeping he same engine conditions helps, but cruise control aims to kEep the same speed instead. On hilly routes in particular cruise control will do much worse as it accelerates, coasts, and brakes to keep the set speed.
__________________
Appreciate 0
      02-14-2012, 03:27 AM   #10
RhoXS
Second Lieutenant
15
Rep
233
Posts

Drives: 2002 Z3 3.0i + 2016 X3 28i
Join Date: Jan 2012
Location: Florida

iTrader: (0)

Quote:
Originally Posted by x3heh View Post
The less acceleration the better for fuel economy. Cruise control helps a lot.
I completely agree. A higher rate of acceleration will burn more gas for the same change in speed than a lower acceleration rate. Slowly accelerating in as high gear (low engine rpm) as possible will give the best fuel economy. Petal to the metal acceleration makes Exxon-Mobile stockholders smile.

Back in the 50s and 60s there used to be a yearly long distant economy run that was always a feature article in Popular Mechanics. I think it was called the "Mobile Economy Run". They would take, if I remember correctly, a stock automobile and drive it across country and achieve some truly phenomenal average mpg numbers, much greater than anything normally achieved. I remember reading how they did it. They used very well practiced and disciplined drivers that held their entire bodies and accelerator foot extremely still. I read an article by one driver that even stated talking would reduce his economy numbers. If they slowed going up a hill they did not compensate by giving more gas. They anticipated stopping and used the brakes as little as possible, kept all the windows up for the least wind resistance, accelerated as slow as they reasonably could, etc. So, yes, using cruise control would not do as well here because the cruise control would add fuel and accelerate to maintain the preset speed.

Of course, a "however" is now coming with respect to the cruise control. However, if you carefully watch the speed of the average driver without cruise control, you will notice the speed varies quite a bit over a band of a few mph. This constant speeding up (accelerating) and slowing down is noticeably less fuel efficient than the constant speed maintained by a good cruise control. So, unless you want to drive ridged still, not talk to your passenger (that might be a good thing), and have practiced maintaining complete control over your accelerator foot, cruise control will allow you to use less gas.
Appreciate 0
      02-14-2012, 04:08 AM   #11
paul386
Second Lieutenant
18
Rep
274
Posts

Drives: 2011 X3 35i Jet Black
Join Date: Oct 2009
Location: Florida

iTrader: (0)

Quote:
Originally Posted by RhoXS View Post
I completely agree. A higher rate of acceleration will burn more gas for the same change in speed than a lower acceleration rate. Slowly accelerating in as high gear (low engine rpm) as possible will give the best fuel economy. Petal to the metal acceleration makes Exxon-Mobile stockholders smile.

Back in the 50s and 60s there used to be a yearly long distant economy run that was always a feature article in Popular Mechanics. I think it was called the "Mobile Economy Run". They would take, if I remember correctly, a stock automobile and drive it across country and achieve some truly phenomenal average mpg numbers, much greater than anything normally achieved. I remember reading how they did it. They used very well practiced and disciplined drivers that held their entire bodies and accelerator foot extremely still. I read an article by one driver that even stated talking would reduce his economy numbers. If they slowed going up a hill they did not compensate by giving more gas. They anticipated stopping and used the brakes as little as possible, kept all the windows up for the least wind resistance, accelerated as slow as they reasonably could, etc. So, yes, using cruise control would not do as well here because the cruise control would add fuel and accelerate to maintain the preset speed.

Of course, a "however" is now coming with respect to the cruise control. However, if you carefully watch the speed of the average driver without cruise control, you will notice the speed varies quite a bit over a band of a few mph. This constant speeding up (accelerating) and slowing down is noticeably less fuel efficient than the constant speed maintained by a good cruise control. So, unless you want to drive ridged still, not talk to your passenger (that might be a good thing), and have practiced maintaining complete control over your accelerator foot, cruise control will allow you to use less gas.
I'm not sure I agree. Yes faster acceleration requires a larger force from the engine but it requires it for less time. Everything is a compromise. Lowest possible engine speed is not the most efficient setting possible.

With engine speed you are balancing the inertia effects of the engine parts accelerating,the piston friction, and combustion efficiency with the heat lost through the cylinder walls. To wager a guess, I would imagine that the most efficient RPM for your engine to operate is slightly higher than what you might expect. The designers also had to balance noise and engine lifespan into the equation as well.
Appreciate 0
      02-14-2012, 10:51 AM   #12
x3heh
Private
United_States
1
Rep
79
Posts

Drives: Honda
Join Date: Jan 2012
Location: United States

iTrader: (0)

Simply put you want as much constant speed with as little acceleration for best fuel economy.
Appreciate 0
      02-14-2012, 12:12 PM   #13
April1
Love All
April1's Avatar
United_States
170
Rep
740
Posts

Drives: X3 28i 2011
Join Date: Nov 2011
Location: San Francisco

iTrader: (2)

Driving speed is every ones preference, minimize applying brakes to get best fuel economy. maintain distances and use brake only when needed. With this technique, I have even got 28 mpg in city driving.
Appreciate 0
      02-14-2012, 12:50 PM   #14
troggy1969
Lieutenant
United Kingdom
54
Rep
554
Posts

Drives: 335D MSport 2016
Join Date: Sep 2011
Location: Ludlow

iTrader: (0)

Garage List
If you drive a petrol 4x4 should you really bother about economy. If you did you would drive a Prius!
Appreciate 0
      02-14-2012, 02:55 PM   #15
richardew
Second Lieutenant
richardew's Avatar
9
Rep
272
Posts

Drives: 2011 x335i
Join Date: Aug 2008
Location: suburban Phila

iTrader: (0)

Here is my real world fuel mileage. I've had the car 5 1/2 months, driven 15269.2 miles and used 723.325 gallons of 93 octane and once in a while 92 octane. Overall 21.1 mpg. My best for a tank was 24.03, 364.6 miles, 15.172 gal. These are not estimates from the vehicle but calculated from the amount of fuel I had to replace in the tank. I don't drive slowly though.
__________________
Rich

'96 911 C4 Cab (276,000 1 owner smiles)
'11 X3 35i Black Sapphire/Beige,SAP, CP, CWP, DHP, PREM, TECH, Sat radio, Hi Fi, BMW apps, smartphone, 19" w/mixed performance tires.
Appreciate 0
      02-14-2012, 03:19 PM   #16
Want the thrill
Lieutenant Colonel
326
Rep
1,560
Posts

Drives: 2022 X4M40i, retired ‘11 X3
Join Date: Mar 2011
Location: MI

iTrader: (2)

Garage List
2022 X4M40i  [10.00]
Quote:
Originally Posted by richardew View Post
Here is my real world fuel mileage. I've had the car 5 1/2 months, driven 15269.2 miles and used 723.325 gallons of 93 octane and once in a while 92 octane. Overall 21.1 mpg. My best for a tank was 24.03, 364.6 miles, 15.172 gal. These are not estimates from the vehicle but calculated from the amount of fuel I had to replace in the tank. I don't drive slowly though.
Wow, you sure do drive a lot! I've had my X3 7 months and have 4,500 miles on it!
Appreciate 0
Post Reply

Bookmarks

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -5. The time now is 11:04 AM.




xbimmers
Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
1Addicts.com, BIMMERPOST.com, E90Post.com, F30Post.com, M3Post.com, ZPost.com, 5Post.com, 6Post.com, 7Post.com, XBimmers.com logo and trademark are properties of BIMMERPOST