Use GPS to record constant speed rolling down hill.
Using GPS Rolling Resistance Calculator we calculate the drag power required to maintain various speeds. The data isn't great but there is an obvious correlation.
This graph is based on a number of hills on Auckland's north shore, and a few trips over the Harbour Bridge.
We see that the green mini uses about 11kW at 95km/h. At 77km/h it uses about 6kW.
An outstanding issue is that I don't know if the velocity provided by the GPS includes the vertical component. Since the vertical component is approximately 4km/h, the combined velocity vector is all but unchanged (95 + 4 = 95.08), so I'm going to ignore this.
I also measured my other car, which has different tires, and known to be poor wheel alignment (the rear end is bent). I haven't weighed this car, so I've guessed that it is the same as the Green mini. This is likely to be an overestimate. This data was collected with a passenger, hence the greater actual weight.
This graph is based on 8 trips over the harbour bridge on a fairly still night with each crossing within a few minutes of the last. I expect that the same road profile and still air contribute to the reduced noise in this data.