How to determine the CDA of your car:
There are three possible ways to determine the CDA
of your car:
•
Find specifications for your car’s CDA
•
Estimate your car’s CDA
•
Calculate your car’s CDA using a coastdown test
Details on each method follow.
1
Manufacturer’s specifications or other research
Very rarely do manufacturers list the aerodynamic
qualities of their cars, unless those qualities of the car
are exceptional.
However, sometimes you get lucky. One of our
test vehicles was an Audi S4. Searching on Google, we
quickly found specifications for the faster European
RS4, which is virtually identical in size and shape to
the S4.
Note: If you find specifications for foreign cars
they usually list frontal area in M
2
, or square meters.
Multiply those figures by 10.76 to convert to square
feet.
Try searching for the terms “drag coefficient” or
“frontal area” and the make of your car.
Also, we will be compiling a list of different ve-
hicles and their CDA values and we will post that infor-
mation on our website, so be sure to check that as well.
23
22
2
Estimate your vehicle’s CDA
Use the following examples to estimate the CDA for
your vehicle.
Honda Insight
.25 CD x 20.4 sq. ft. = 5.1 CDA
Corvette C5
.29 CD x 21.3 sq. ft. = 6.2 CDA
Porsche 911
.32 CD x 19.4 sq. ft. = 6.2 CDA
Audi S4
.31 CD x 21.9 sq. ft. = 6.8 CDA
Honda Civic
.34 CD x 21.0 sq. ft. = 7.1 CDA
Minivan or SUV .40 CD x 30.0 sq. ft. = 12.0 CDA
Pickup or van
.45 CD x 35.0 sq. ft. = 15.8 CDA
Modern passenger car CD’s are usually .30 to .35.
Van, SUV and truck CDs are usually .35 to .45.
3
Calculate your vehicle’s CDA
You can determine the correct CDA number by using a
“coastdown” test.
•
On a flat road, on a day with no wind, accelerate
to slightly above 60 mph.
•
Put your car in neutral (whether your car has an
automatic or manual transmission, it is important
to put the car in neutral).
•
Measure the time it takes your car to decelerate
from exactly 60 mph to exactly 50 mph.
•
Perform this test at least three times in each
direction, to make sure your results are relatively
consistent.
•
Average your times, then use the table to the right
to determine the CDA factor.
•
Multiply your car’s weight times the CDA factor
from the table to determine your car’s CDA.
EXAMPLE: Find the CDA of a 4000 pound car that takes
17 seconds on average to coast from 60 to 50 mph.
Look at the table. For 17 seconds, the factor is .001824.
Multiply that times 4000 pounds to get the car’s CDA.
4000 x .001824 = 7.296
7.296 is the car’s approximate CDA.
Multiply your
car’s weight by
this number to
calculate your
CDA.
10.0
.004305
10.5
.004018
11.0
.003757
11.5
.003519
12.0
.003301
12.5
.003100
13.0
.002915
13.5
.002743
14.0
.002584
14.5
.002435
15.0
.002297
15.5
.002167
16.0
.002046
16.5
.001932
17.0
.001824
17.5
.001723
18.0
.001627
18.5
.001537
19.0
.001451
19.5
.001370
20.0
.001293
The average time
it takes your car
to coast from
60 mph to 50 mph
(in seconds)
