OK, let's take another look at the true aero efficiency increase in the Insight due to the tail extension.
Shown below are two sets of theoretical data based on real world gas consumption during a 1.3 mile run on a reasonably flat road, with the conditions shown in the previous post.
The group of data on the left side is without the tail and on the right, with the tail.
As before, we have assumed that the mix between rolling and aero drag is identical at a speed of 45 mph. So with this in mind, the Crr is arbitrarily set to 0.5 and the aero is also set to 0.5. At a speed of 45 mph, this gives us a total drag component of 1.0 when adding the Crr and Aero components.
Since the gas mileage test was run at 56 mph and not 45, we have identified this data in the blue row, as shown on the left. Notice that the total drag without the tail is indicated by the red box as 1.39...
Now we move to the data on the right side, and this is where the theoretical part begins.
If we take the total drag number without the tail at 1.39 and reduce it by 10.27%, which is the decrease in fuel consumption due to the tail, then we now have a target drag for the car with the tail added, which is 1.25....
This 1.25... number is again indicated by the red box, and this is our target total drag that we are looking for. So the Aero drag due to the tail is now reduced until we get the total drag to be 1.25... at 56 mph. Once we have reached this point by iteration, we now know exactly the mix of Crr and Aero it takes to get a reduction in total drag of 10.27%.
So how much of an Aero drag decrease did the tail make on the car?
The "Diff" column shows a consistent 18.5% decrease in Aero drag due to the tail.
So with this
limited distance fuel consumption test, we tentatively can say the tail offered almost a 20% decrease in Aero drag.
Jim.