I do not think the speedo will be accurate enough, especially at such low speeds and small speed change. Would be better to start from a standstill on a small incline and coast down on to a level section until you stop and work off the distance covered. Balancing will be a bit more difficult but should be doable.
In saying that, even if you can get consistent results how will you relate them to the real world of how it affects the performance of your bike on a normal ride?