A 20% reduction in drag = a 10% increase in fuel economy ?

[UltArc]

The math I am not sure about, but it seems to be a quadratic equation rather than linear.

I do not THINK it is as easy to compute as that. I am not an expert, nor do I know too much at all about the subject, I just can't believe it to be so easy.

[KamperBob]

Originally Posted by Cd

I'm miserably bad at math, so please check my numbers .

.31 Cd minus 45% equals .1705
45 MPG plus 110 % equals 94.5 MPG

So a reduction of 45% in drag equals a 110% increase in fuel economy ?

Your math misses other variables you don't know: rolling resistance of tires, engine mods, driving habits, etc. A-B testing risks time correlation. A-B-A testing rechecks baseline. However, once a car is supermodded another "A" run isn't very likely. That's not necessarily a bad thing. Real gains are sticky. Why undo successful mods to prove they helped? Human nature... (shrug)

[Daox]

That rule of thumb is incredibly general. I don't believe it was intended with any specific speed relation. So, basically it says reduce Cd by 20% and gain 10% FE. Obviously you'll see better FE gains at higher speeds and less at lower speeds.

[MetroMPG]

You can also plug and play with the numbers here:

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

[MetroMPG]

Originally Posted by Cd

45 MPG plus 110 % equals 94.5 MPG

Also, why did you choose 45 MPG as the "original" 65 MPH fuel consumption figure? There's no rule that says the EPA figure necessarily equals MPG @ 65 MPH. Depending on the car, real world MPG at a steady 65 could be higher or lower than the highway rating.

[Sven7]

I recall Phil saying the half rule is based on 55mph steady state driving. Thus, basjoos's 65mph figure would be on a different part of the graph, so to speak. Also, drag does not increase linearly, as UltArc pointed out. For example, when you go 10% faster there may be 12% or 15% more drag.

So, if you want to calculate 55mph cruising mpg of a future mod you can use the half rule and get it close enough. If you're looking at other speeds it will not work correctly.

[Diesel_Dave]

It's all speed dependent. I found these graphs from a Cummins white paper. It's geared towards semi trucks, but it illustrates the principle.

Here's the first plot. It shows where the power goes vs vehicle speed:
The red portion is the power required to overcome the aero drag. Obviously, the % benefit you're going to see is dependent on the vehicle speed.

This second plot (from the same white paper) shows % mpg benefit vs % aero improvement for different duty cycles:
Note that a low speed duty cycle (inter-city) shows very limited benefit from aero aids vs a high speed duty cycle (interstate) show substantial mpg benefits.

[aerohead]

Originally Posted by Cd

So if I take my 1993 Civic hunchback with a stock Cd of .31 and drop the Cd by 60% down to .124 , then my fuel economy will rise by 30% ?

42 MPG highway + 30% = 54.6 MPG ?


( yes - I completely FAIL at math )

*If you drop the Cd from 0.32,to 0.124 you have only 38% of your original drag remaining.
*This required a 62% drag reduction to get there.
*At 55 mph,a 10% delta-Cd = 5% delta-mpg.
*Dividing 62% by 10% leaves 6.2 X 5%,or 31% mpg increase.
*So,1.31 X 42= 55.02 MPG @ 55 mph.
------------------------------------------------------------------------
*If you get 42 mpg at 70 mph you pick up 6% mpg per 10% delta-Cd
*So 62% divided by 10% = 6.2
*And 6.2 X 6% = 37%
*Then 1.37 X 42 = 57.54 mpg @ 70 mph.

[aerohead]

Originally Posted by MetroMPG

Also, why did you choose 45 MPG as the "original" 65 MPH fuel consumption figure? There's no rule that says the EPA figure necessarily equals MPG @ 65 MPH. Depending on the car, real world MPG at a steady 65 could be higher or lower than the highway rating.

The original Federal Test Cycle for the Highway portion of the EPA testing included a maximum 60 mph velocity,and averaged only 49.6 mph for the entire test protocol.
At least two additional tests have been added now which include acceleration to higher speeds.
I'd have to visit the EPA Mobile Sources website to know what they're doing currently.
There would be a date in which testing 'switched over' and newer estimates would reflect the more modern evaluations.
Better dynos are allowing more 'real world' type testing.

[aerohead]

Originally Posted by Daox

That rule of thumb is incredibly general. I don't believe it was intended with any specific speed relation. So, basically it says reduce Cd by 20% and gain 10% FE. Obviously you'll see better FE gains at higher speeds and less at lower speeds.

I believe that this is how it is,based on some of the published work.
And for 'noodling' purposes,I think it's okay to plot values for 55 mph and 70 mph,draw a line to connect them,then interpolate or extrapolate the percentage relationship to any velocity of interest as long as we don't stray too far from the known 55 and 70 data points.
If members will establish their baseline data at one or the other,or both velocities,it will make for a higher confidence when questimating.