Puzzled about MPG% vs. .Cd

[Nerds laugh at me]

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.

[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.

[Nerds laugh at me]

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 ?

[trebuchet03]

Originally Posted by Nerds laugh at me

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

[thisisntjared]

Originally Posted by trebuchet03

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!

[trebuchet03]

Originally Posted by thisisntjared

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!

[thisisntjared]

Originally Posted by trebuchet03

I Love your cars!

... thanks

i have to admit i can't tell if you are being facetious.

[trebuchet03]

Originally Posted by thisisntjared

... 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

[thisisntjared]

Originally Posted by trebuchet03

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.

[brucepick]

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).