04-26-2012, 07:47 PM
|
#1 (permalink)
|
|
EcoModding Apprentice
Join Date: Apr 2012
Location: Charlotte, NC
Posts: 134
Thanks: 143
Thanked 110 Times in 47 Posts
|
Relationship between MPG increase and drag decrease?
Has anyone run across a formula or rule of thumb that correlates increases in MPG to drag decrease? Yes, there are a lot of other factors that combine to generate drag, but is there even a rough rule of thumb?
|
|
|
|
 Today
|
|
|
|
Other popular topics in this forum...
|
|
|
|
|
04-26-2012, 08:00 PM
|
#2 (permalink)
|
|
Master EcoModder
Join Date: Dec 2011
Location: Boise Idaho
Posts: 842
Thanks: 39
Thanked 89 Times in 69 Posts
|
the mpg calculator on this site is the granddaddy of all.
|
|
|
|
|
04-26-2012, 08:07 PM
|
#3 (permalink)
|
|
(:
Join Date: Jan 2008
Location: up north
Posts: 12,762
Thanks: 1,585
Thanked 3,555 Times in 2,218 Posts
|
aerohead has posted many a time about a rule of thumb for fe increase/drag decrease. But I don't remember it just now. :/
|
|
|
|
|
04-27-2012, 05:57 AM
|
#4 (permalink)
|
|
Eco-ventor
Join Date: Oct 2010
Location: sweden
Posts: 1,631
Thanks: 74
Thanked 702 Times in 445 Posts
|
~2/3 for highway.
__________________
2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)
|
|
|
|
|
04-27-2012, 01:19 PM
|
#5 (permalink)
|
|
Master EcoModder
Join Date: Aug 2011
Location: Warren, MI
Posts: 2,456
Thanks: 782
Thanked 668 Times in 410 Posts
|
MPG % increase is half of aero % increase.
__________________
He gave me a dollar. A blood-soaked dollar.
I cannot get the spot out but it's okay; It still works in the store
|
|
|
|
|
The Following User Says Thank You to Sven7 For This Useful Post:
|
|
|
04-27-2012, 01:46 PM
|
#6 (permalink)
|
|
Master EcoModder
Join Date: Dec 2008
Location: Southern WI
Posts: 827
Thanks: 101
Thanked 560 Times in 191 Posts
|
Quote:
Originally Posted by Sven7
MPG % increase is half of aero % increase.
|
And it's nice to explain in more detail why this is....
I have done numerous coast down tests with the Insight and Crr and Cd are about equal at 35 mph or so.
This means if you change the aero of your car and make it 100% more slippery to the wind, it will gain you 50% more gas savings, because while the aero part improved 100%, the Crr part stayed the same. Then when you add the new Cd and the old Crr, you have a 50% net gain.
Jim. |
|
|
|
|
04-27-2012, 06:19 PM
|
#7 (permalink)
|
|
Master EcoModder
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 15,883
Thanks: 23,957
Thanked 7,219 Times in 4,646 Posts
|
Caveat
Quote:
Originally Posted by Sven7
MPG % increase is half of aero % increase.
|
I just wanted to say that the 50% rule applies at 55 mph.
At 70 mph,a 10% drag reduction will net a 6% mpg improvement.
If you're driving in Wyoming or the Trans-Pecos region of Texas,at 80 mph,it will be even more. |
|
|
|
|
The Following User Says Thank You to aerohead For This Useful Post:
|
|
|
04-27-2012, 06:21 PM
|
#8 (permalink)
|
|
Master EcoModder
Join Date: Jan 2012
Location: United States
Posts: 1,756
Thanks: 104
Thanked 407 Times in 312 Posts
|
Quote:
Originally Posted by 3-Wheeler
And it's nice to explain in more detail why this is....
I have done numerous coast down tests with the Insight and Crr and Cd are about equal at 35 mph or so.
This means if you change the aero of your car and make it 100% more slippery to the wind, it will gain you 50% more gas savings, because while the aero part improved 100%, the Crr part stayed the same. Then when you add the new Cd and the old Crr, you have a 50% net gain.
Jim.
|
Aero drag completely eclipses rolling resistance at higher speeds though. The reason is because the lower the load on your engine (generally, in the bottom 50-80% of torque output depending on forced induction or cams and stuff), the less efficiently it runs. |
|
|
|
|
04-27-2012, 06:31 PM
|
#9 (permalink)
|
|
Master EcoModder
Join Date: Aug 2011
Location: Warren, MI
Posts: 2,456
Thanks: 782
Thanked 668 Times in 410 Posts
|
Quote:
Originally Posted by 3-Wheeler
And it's nice to explain in more detail why this is....
I have done numerous coast down tests with the Insight and Crr and Cd are about equal at 35 mph or so.
This means if you change the aero of your car and make it 100% more slippery to the wind, it will gain you 50% more gas savings, because while the aero part improved 100%, the Crr part stayed the same. Then when you add the new Cd and the old Crr, you have a 50% net gain.
Jim.
|
Thanks, I didn't know how the rest broke down because honestly I can't do a lot to change it. Now, aero, that's something to easily improve upon 
__________________
He gave me a dollar. A blood-soaked dollar.
I cannot get the spot out but it's okay; It still works in the store
|
|
|
|
|
