What Do These Numbers Mean?

[Ekh]

In his interview, Jeff Johnson (VP Engineering) mentioned two values: coefficient of drag (c/d) and rolling resistance. He said c/d is .3, and rolling resistance is .008 (crediting Continental's redesign of the wheel and tires for achieving this).

Asking "OK, just how good IS the Elio?" I checked out what these values mean. c/d "is a unit-less value that denotes how much an object resists movement through a fluid such as water or air." It is affected by shape, frontal area, and things like mirrors or front bumpers lousing up the air flow.

Coefficient of drag (c/d)

C/D is discussed, along with a very long list of cars showing their c/d here: https://en.wikipedia.org/wiki/Automobile_drag_coefficient

Rolling resistance is the sum of forces that keep a vehicle from rolling forward. Tire design, wheel size, and road surface all affect this. Technical discussion here: http://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html

With a c/d of 3 Elio is looking pretty darn good.

Rolling Resistance

From a technical piece on The Tire Rack's site, we learn that

During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%, air drag is about 5% and tire rolling resistance is about 15%.

Overcoming inertia no longer plays an appreciable role in the vehicle's resistance during steady speed highway driving. For those conditions it is estimated that driveline friction is about 15%; air drag is about 60% and tire rolling resistance represent about 25%.
​

So if you can make a significant reduction to rolling resistance, your highway mileage will greatly improve (all other things like aerodynamic drag being equal). Tire rolling resistance has an effect on vehicle fuel consumption estimated to range from about 4% during urban driving to 7% during highway driving.

Measuring rolling resistance involves spinning tires against a large drum which measures the resistance directly. The problem is that you can't compare groups of tires against each other; you can have two grossly different vehicles (a Suburban and an Elio, for instance) both using tires with the same rolling resistance, yet with completely different actual fuel economy results. From The Tire Rack:

Comparing Rolling Resistance Coefficients only allows comparing tires within a single size. Tire Rolling Resistance Coefficient does not allow comparing different sized tires.​

So saying the Elio's .008 is the same rolling resistance as a bicycle tire on rough paved road (which is the case) isn't actually very helpful. The inability to compare tires of different sizes using RR makes the coefficient for rolling resistance a difficult number for consumers to use, but it's of great use to tire and automotive designers trying to squeeze 84 miles out of every gallon of gas!

 

[johnsnownw]

For the c/d, usually it's listed to the hundredths, like the Bolt is .32. I think .30 is definitely possible, so it may be that. However, I sort of expected it to be below .3...but the exposed suspension must be impacting it more than it seems it would.

 

[Ekh]

For the c/d, usually it's listed to the hundredths, like the Bolt is .32. I think .30 is definitely possible, so it may be that. However, I sort of expected it to be below .3...but the exposed suspension must be impacting it more than it seems it would.

If you check out the first link in my post, you'll see that c/d is expressed there as an integer followed by one or two decimal points, not as 0.XX.

 

[johnsnownw]

They are listed as 0.xx...in the table from your link.

 

[W. WIllie]

The G1 Insight is .25, so no reason the ELIO shouldn't be lower. (Of course the G1 has aero-panels.)

 

[Ekh]

They are listed as 0.xx...in the table from your link.

What in heck was I looking at?

 

[Ekh]

Should be... jeff johnson stated plainly it is .3. Except he didn't say "point"

 

[floydv]

0.3 isn't bad, but it's hard to believe the BMW 3-Series has better aero than Elio. I would've expected a bit better with the "half the car/twice the mileage" credo.

 

[Gregw]

.3 is the same as a Ford Taurus, somehow I think he miss-spoke.

 

[electroken]

If you check out the first link in my post, you'll see that c/d is expressed there as an integer followed by one or two decimal points, not as 0.XX.

The coefficient of drag for any car will be less than 1. Even a slippery barn should come in around 1.