I can use my butt-o-meter on my daily commute
if you know well a portion of road you can see how the comportment is different. On a couple of small down slopes my car was slowing down and now it's accelerating. Also on a bigger down slope when I leave the top at a given speed, I arrive at a bigger speed at the bottom with the rear skirts than without.
As Pale said, coast down testing is a possibility. You may use the calc on
IWillTry.org (see "Measure the drag coefficient of your car") if you want to calc your Cd.
I have also the idea to test the recording of the signal from a gps receiver while doing coast down and measure an average slow down in m/s/s with & without the mod. Currently time is missing
not the ideas...
Denis.
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