Puzzled about MPG% vs. .Cd - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

Reply
 
Thread Tools Display Modes
 
Old 09-13-2007, 07:58 PM   #1
Registered Member
 
Join Date: May 2007
Posts: 200
Country: United States
Puzzled about MPG% vs. .Cd

When it is stated that an automobile can get a certain percentage greater fuel economy by changing its aerodynamics, does this percentage change with the cars MPG average ?
Example : Lets say that I have a car that gets 75 MPG and I change the aero by 50%. Supposedly the MPG will then increase buy a certain percent as a result.
Now lets say I have a truck that gets 4 MPG and do the same - change the .Cd by 50 %.
I would think that the MPG increase would hardly be noticeable, whereas the car getting 75 MPG would see an increase of several miles per gallon.

So I suppose what I am asking is if this is if the math is scaleable

( or something like that )

It's late and I need sleep if none of this is making any sense.
__________________

Nerds laugh at me is offline   Reply With Quote
Old 09-13-2007, 09:05 PM   #2
Registered Member
 
trebuchet03's Avatar
 
Join Date: Mar 2007
Posts: 812
Country: United States
Send a message via AIM to trebuchet03
So the thing about cD is... It's unit less. So there's no way to correlate a % change in cD (a unit less coefficient) with mpg (a unit of measurement) - at least not directly.

So, you can correlate engine load with resistance -- which is related to FE. Now, aero enhancement shows more gains at higher speeds - and here comes the fun/confusing part. Given that aero resistance isn't linear with respect to velocity, engine load v. resistance (with respect to velocity) is also not a linear relationship.
__________________

__________________
Time is the best teacher. Unfortunately it kills all its students.


Bike Miles (Begin Aug. 20 - '07): ~433.2 miles

11/12
trebuchet03 is offline   Reply With Quote
Old 09-14-2007, 01:05 PM   #3
Registered Member
 
Join Date: May 2007
Posts: 200
Country: United States
Sorry. You smart. Me dumb.
I'll try and understand.

In other words, what you are saying is that there is not really a set figure that you can rely on to estimate your .Cd based on the change in MPG ?
Nerds laugh at me is offline   Reply With Quote
Old 09-14-2007, 02:20 PM   #4
Registered Member
 
trebuchet03's Avatar
 
Join Date: Mar 2007
Posts: 812
Country: United States
Send a message via AIM to trebuchet03
Quote:
Originally Posted by Nerds laugh at me View Post
Sorry. You smart. Me dumb.
I'll try and understand.

In other words, what you are saying is that there is not really a set figure that you can rely on to estimate your .Cd based on the change in MPG ?
No, there's no set figure given the variety in vehicle design... But you can make one indirectly for you car You can make a cd v. mpg trend for you car. You'll need to know the mpg at a specific velocity of your car and the cD of your car. Then, you need to find another mpg at a different cD. And come up with a few different data points. Then draw a trend line around that data set. Assuming all extra values equal during your testing, you should have a quasi reliable relationship for one specific velocity. If you repeat the process at different velocities, you can make a cD v. mpg v. velocity map.

I think that's about right.... Perhaps I should do that one day I think it would look pretty sweet (intuitively, it should make a slope from three corners to the opposite corner)

And I am far from "smart" - flattering, but not true

-----
Measure the cD of your Car
__________________
Time is the best teacher. Unfortunately it kills all its students.


Bike Miles (Begin Aug. 20 - '07): ~433.2 miles

11/12
trebuchet03 is offline   Reply With Quote
Old 09-14-2007, 05:56 PM   #5
|V3|2D
 
thisisntjared's Avatar
 
Join Date: Mar 2006
Posts: 1,186
Country: United States
Send a message via AIM to thisisntjared
Quote:
Originally Posted by trebuchet03 View Post
And I am far from "smart" - flattering, but not true
well you are in luck today! i am smart and what you said in the rest of your post is true!
__________________
don't waste your time or time will waste you
thisisntjared is offline   Reply With Quote
Old 09-14-2007, 06:49 PM   #6
Registered Member
 
trebuchet03's Avatar
 
Join Date: Mar 2007
Posts: 812
Country: United States
Send a message via AIM to trebuchet03
Quote:
Originally Posted by thisisntjared View Post
well you are in luck today! i am smart and what you said in the rest of your post is true!
I Love your cars!
__________________
Time is the best teacher. Unfortunately it kills all its students.


Bike Miles (Begin Aug. 20 - '07): ~433.2 miles

11/12
trebuchet03 is offline   Reply With Quote
Old 09-14-2007, 07:33 PM   #7
|V3|2D
 
thisisntjared's Avatar
 
Join Date: Mar 2006
Posts: 1,186
Country: United States
Send a message via AIM to thisisntjared
Quote:
Originally Posted by trebuchet03 View Post
I Love your cars!
... thanks

i have to admit i can't tell if you are being facetious.
__________________
don't waste your time or time will waste you
thisisntjared is offline   Reply With Quote
Old 09-14-2007, 08:29 PM   #8
Registered Member
 
trebuchet03's Avatar
 
Join Date: Mar 2007
Posts: 812
Country: United States
Send a message via AIM to trebuchet03
Quote:
Originally Posted by thisisntjared View Post
... thanks

i have to admit i can't tell if you are being facetious.
I'm thinking of a quote... It's something like "Puns are not funny"

http://www.smartusa.com/smart-car-fortwo.aspx
__________________
Time is the best teacher. Unfortunately it kills all its students.


Bike Miles (Begin Aug. 20 - '07): ~433.2 miles

11/12
trebuchet03 is offline   Reply With Quote
Old 09-15-2007, 07:47 AM   #9
|V3|2D
 
thisisntjared's Avatar
 
Join Date: Mar 2006
Posts: 1,186
Country: United States
Send a message via AIM to thisisntjared
Quote:
Originally Posted by trebuchet03 View Post
I'm thinking of a quote... It's something like "Puns are not funny"

http://www.smartusa.com/smart-car-fortwo.aspx
ah, well my father always said that puns are the lowest form of humor unfortunately i am full of them... usually unintentional.
__________________
don't waste your time or time will waste you
thisisntjared is offline   Reply With Quote
Old 09-15-2007, 06:47 PM   #10
Registered Member
 
brucepick's Avatar
 
Join Date: Feb 2007
Posts: 722
Country: United States
Location: Connecticut
My .02 on the op's question.

I'm with trebuchet03 - there's no direct correlation between FE and drag reduction. As for the rest of t03's analysis - I think he's ahead of me in the detailed smarts department.

Improved aero will help you most at higher speeds. Again, how much it helps at any given speed will vary with different cars and drivers. But if you're doing lots of highway driving or even 45-55 mph cruising, aero drag reduction should help your FE. At those speeds, a lot of the work your engine does is simply overcoming air resistance/drag.

On the other hand, if you're doing mostly short hops or driving around town where you stop and accelerate all the time, mostly between 10-30 mph, then aero improvements will probably help you very little if at all. For that situation,you might want a block heater (for shorter warmup time), and weight reduction, small engine, and stick shift for reduced power consumption while accelerating (and engine off neutral coasting - needs a stick).
__________________

__________________
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
brucepick is offline   Reply With Quote
Reply

Thread Tools
Display Modes

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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
mpg display for carb engine retrorocket General Fuel Topics 10 05-27-2007 09:48 PM
the ZENN neighbourhood electric vehicle MetroMPG Electric and Solar powered 9 03-23-2007 10:48 PM

Powered by vBadvanced CMPS v3.2.3


All times are GMT -8. The time now is 03:29 PM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.