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The downhill test might work if you can do it in neutral - not sure that is possible in the Prius. Otherwise, regen braking issues, etc.
Depending on your speed limits / interest of the law, I'd probably try to test fuel consumption over a short distance (eg 1-2km) on a flat, straight road at the highest possible constant speed eg 130 km/h+.
With either, do windows up/down to ensure your are getting real data.
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