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      11-02-2012, 02:07 AM   #1
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Speed in relation to MPG

So I drove to Vegas this past weekend and just wanted to share my MPG's. I have an F30 335i with the auto. Drove from San Diego and averaged 77 mph, trip took 4 hours and 40 mins, and averaged 33.7 MPG this was on comfort mode with one passenger and two small suitcases. I guess what I wanted to see is if anyone else has looked at what the most efficient speed would be for gas economy. On the way back I wanted see what the eco-mode would do, but it seemed to not make a difference. Best speed to MPG i found was 80 MPH =34MPG. going 75 MPH averaged about 32mpg. I made sure that it was flat and not going up or down any inclines. I guess i got a little board. GF doesn't like going over 80's but if anyone has any input at higher speeds I would like to hear.
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      11-02-2012, 03:42 AM   #2
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Lower speeds are better for economy as your own test shows. Car has to do less work to push through the air.
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      11-02-2012, 04:14 AM   #3
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Quote:
Originally Posted by Dabaitu View Post
Lower speeds are better for economy as your own test shows. Car has to do less work to push through the air.
Agree, can't defy the laws of physics. Increased speed means more air resistance, needs more power, uses more energy, burns more fuel.

When cruising and you have to slow down to go through restricted zones, mpg improves, particularly if you can stay in the highest gear... simples.

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      11-02-2012, 06:30 AM   #4
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Actually, according to the original poster he saw better fuel efficiency at 80 than at 75mph. Ghinnes, I am somewhat inclined to agree with your observations.

In my old n52, I was seeing better mileage above the speed limit. It seems these cars are calibrated to run at specific speeds.
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      11-02-2012, 07:06 AM   #5
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car manufacturers should publish this most economical speed (all things being equal).
I think it would be a great benefit to owners. A lot of variables involved here including the engine, gearbox , temperature and loads on the negine from gadgets such as a/c, heated seats etc.
Aircraft have V speeds which define the speed thigs occur or limitations and perhaps cars should have something similar?

I've a 530d and for me it this best economical speed seems to be around 50mph but is difficult to measure.
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      11-02-2012, 07:39 AM   #6
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Quote:
Originally Posted by AP View Post
car manufacturers should publish this most economical speed (all things being equal).
I think it would be a great benefit to owners. A lot of variables involved here including the engine, gearbox , temperature and loads on the negine from gadgets such as a/c, heated seats etc..
Totally! After all, every engine must have a sweet spot where they extract the greatest torque to fuel ratio. That engine speed in top gear will always be the most economical cruise
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      11-02-2012, 07:48 AM   #7
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They way I look at it is that MPG is actually more directly related to throttle position than speed.
If you were driving into a wind there would be more resistance at 140kph therefore more throttle. If you had a nice tailwind there would be less resistance therefore less throttle.
If you were driving up a hill at 140kph you would need to more power so you the throttle would be more open. If you were going down the hill you would less power so the throttle would be more closed.

When accelerating how fast you accelerate depends on how much you have the throttle open.
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      11-02-2012, 08:27 AM   #8
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I can't seem to get higher than 19 mpg In city. Is this normal? Also, silly question how do u make ur car stay in highest gear? FYI I drive a 328 rwd
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      11-02-2012, 09:32 AM   #9
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Quote:
Originally Posted by AWF30
I can't seem to get higher than 19 mpg In city. Is this normal? Also, silly question how do u make ur car stay in highest gear? FYI I drive a 328 rwd
That seems really low, mine never drops below 23-24 in city driving.
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      11-02-2012, 09:55 AM   #10
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For urban driving, average speed has a high inverse correlation with fuel consumption. On the highway, there are just too many variables (wind, temperature, road surface, etc.) to get a good comparison, even if the traffic allows a steady speed.
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      11-02-2012, 09:57 AM   #11
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Normal from 18 ~ 22 mpg in city driving, depending on different situation in diffrent city.
Usually you can't get higher in a more stops in Los Angeles. If we can keep at the speed of up 40 with less traffic in Eco mode, that will save a lot.
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      11-02-2012, 09:59 AM   #12
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I keep it in Eco-Pro, average 120 miles a day, and get about 30.3-33 mpg.

When I'm driving at 74-75, I get about 33.
80-85, in the 30's.

Bear in mind, I live in CT, so lots of hills and valleys.

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      11-02-2012, 10:14 AM   #13
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Quote:
Originally Posted by AP View Post
car manufacturers should publish this most economical speed (all things being equal).
I think it would be a great benefit to owners. A lot of variables involved here including the engine, gearbox , temperature and loads on the negine from gadgets such as a/c, heated seats etc.
Aircraft have V speeds which define the speed thigs occur or limitations and perhaps cars should have something similar?

I've a 530d and for me it this best economical speed seems to be around 50mph but is difficult to measure.
WhatCar with fuel economy guru Peter de Nayer did tests across a wide range of cars, to see how and when modern engines give the best mpg. The conclusion surprised many, it is still the lowest speed you can drive in top gear. That means less than 40mph for many cars. Certainly not much higher than 40mph in any car they tested.

They tested the cars at constant speeds and at 10mph increments. There is not one car in the test that improves fuel efficiency with speed over 40mph, all the graphs show increased consumption as speed increases.

If anyone has a controlled test where the 80mph figure is better than the 70mph figure, I'd love to see the data. I'm sure Peter De Nayer would appreciate seeing it as well.

I have a graph of a 535i test, (N54 engine) which clearly follows an almost straight line increase of fuel consumption as speed increases, consumption drops about 9mpg (US) between 50 - 80mph.

Just driving at different speeds and reading the OBC is not a sound test. It has to be controlled, or something as small as a 2mph wind speed, head or tail, will completely skew the result.

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      11-02-2012, 11:34 AM   #14
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yeah, i mean my method of testing was by no means scientific. there is just a batch of long stretch of roads that are strait for miles and i would just set cruise and reset the MPG counter and drive 2-3 miles without hitting the brakes or throttle, just letting the computer do its thing. I work on alot of big boats, ships really, we call them cutters, but they have a sweet spot that it is the most effecient as far as rpm's vs speed vs consumption. Every cutter has a chart for the navigator so they can chart it out and manage fuel. I would thing that in 8th gear going under a speed of lets say 50MPH that the car would work harder and burn more fuel to maintain that speed. Also, I really dont think Eco really does a whole lot on long trips. I used it all they way back and it didnt really have an improvement. It was actually a bit less coming in at 31MPG. there are alot of mountains that you cross on the trip but I started out at sea level and close to vegas just on the boarder is sea level again so i think it would cancel elevation out.
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      11-02-2012, 12:39 PM   #15
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Quote:
Originally Posted by AWF30 View Post
I can't seem to get higher than 19 mpg In city. Is this normal? Also, silly question how do u make ur car stay in highest gear? FYI I drive a 328 rwd
What type of gas are you using? I am getting better than that with my 335i in sport mode.
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      11-02-2012, 12:59 PM   #16
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Quote:
Originally Posted by shivaswrath View Post
I keep it in Eco-Pro, average 120 miles a day, and get about 30.3-33 mpg.

When I'm driving at 74-75, I get about 33.
80-85, in the 30's.

Bear in mind, I live in CT, so lots of hills and valleys.

[RWD 335i]
I drive 84 to the mass pike a lot, live in Manchester. I get mid to upper 30's at 80 in eco mode on cruise control. 80 seems to be the best speed or 75-85 which is probably why they don't publish it, if you told us all that 80 was the best speed who would go 65?



-> shivaswrath I replied to you cause I want to know how much you like the M exhaust?! Looking into it and if you drive up this way I would really appreciate meeting up for 5 minutes to hear it in person.
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      11-02-2012, 01:31 PM   #17
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Quote:
Originally Posted by 1.fresh.f30.335 View Post
80 seems to be the best speed or 75-85 which is probably why they don't publish it, if you told us all that 80 was the best speed who would go 65?
Put some car data into a 'HP to speed' calculator, and see the approximate Horse Power needed for 65 and 80mph.

A quick look for the F30 335i auto from carfolio:
Drag coefficient of 0.3
Frontal area of 23.67 sq ft. (2.2)
Weight 3494 lbs.

Approximate HP to run a steady 65mph = 21HP; for 80mph = 34.5HP. That's over 60% more HP to drive faster. Try and find any engine BSFC map that will show a decrease in fuel consumption for a 60% increase in output, within a 1,000rpm increase. Particularly as you are right in the middle of the torque band at both speeds. Don't think anyone is hiding facts about consumption.

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      11-02-2012, 06:29 PM   #18
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Quote:
Originally Posted by ric124 View Post
What type of gas are you using? I am getting better than that with my 335i in sport mode.
91 octane from shell
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      11-02-2012, 08:39 PM   #19
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91 octane from shell
and I am using 93 octane
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      11-02-2012, 08:54 PM   #20
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Quote:
Originally Posted by 1.fresh.f30.335 View Post
I drive 84 to the mass pike a lot, live in Manchester. I get mid to upper 30's at 80 in eco mode on cruise control. 80 seems to be the best speed or 75-85 which is probably why they don't publish it, if you told us all that 80 was the best speed who would go 65?



-> shivaswrath I replied to you cause I want to know how much you like the M exhaust?! Looking into it and if you drive up this way I would really appreciate meeting up for 5 minutes to hear it in person.
Sure! PM me date/time....

Another point...DON'T FILL up with BJ's gas, that impacted my MPG massively....I get about 50-75 miles MORE with Mobil 1, and I'll try Shell out on the next tank.

I know this is kicking up a storm, but quality of gas will impact your MPG significantly in...at least my car.
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      11-02-2012, 10:30 PM   #21
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Quote:
Originally Posted by shivaswrath View Post
Sure! PM me date/time....

Another point...DON'T FILL up with BJ's gas, that impacted my MPG massively....I get about 50-75 miles MORE with Mobil 1, and I'll try Shell out on the next tank.

I know this is kicking up a storm, but quality of gas will impact your MPG significantly in...at least my car.
Does BJ's use 10% Ethanol and the rest 0% Ethanol? Ethanol will impact MPG negatively.
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