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Back to the topic of 'a new way of measuring on-road drag'.
Last night, the fourth and final professional aerodynamicist got back to me about the veracity of my technique. I don't want to name them because they haven't given me permission to do that, but if you look at who reviewed my book, or worked with me on it, you'll get a good idea. They are all top experts in the field.
All the aerodynamicists agreed that the throttle-stop approach to testing changes in drag was technically valid.
However, one aerodynamicist who gave feedback also made the point that any testing methodology where a change in speed is part of the test (as it is with my throttle stop technique), also has other factors introduced - eg a change in tyre diameter, so impacting ride height. However, I am happy that such impacts will be minor when we're talking only a small change in road speed.
He also said that my technique gives only a single data point - ie it gives a drag change at a certain average speed in specific wind conditions. He said what may give a drag reduction in one condition may not actually be the best for the conditions in which the car spends most of its life. I think that's a valid point, that can in part be addressed by doing the throttle-stop testing of a specific modification in lots of different atmospheric conditions before claiming that it works.
Based on my testing, the technique seems to me to be much more accurate than coastdown testing. It won't find tiny changes in drag (without very special equipment, I don't think they are actually measurable with any on-road technique) but those aerodynamic modifications that have the potential to make changes in mileage should be able to be seen. (And perhaps the combined impact of many small modifications may also be measurable.)
So, as far as I can see, based on the expert judgement, and on what I have found in my actual testing, we have a new way of testing for changes in on-road drag.
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