I just test rode my bike with a new Q-matic drive yesterday. Unfortunately I did not listen to my inner self and go with a 62 or 64 tooth sprocket. In a real world scenario the 56 does not apply. The 56 is great for long open roads in the country , and going downhill. 72 tooth sprocket is great for fast stop and go traffic, or pulling a load, and hill climbing, (Real Hills). I read a post by someone stating their top speed with the 72 tooth is 31 mph. The speed with the 56 is above 40 mph with reports of 45 mph and occasional 50 mph. On You Tube I watched a video, The person had a 62 tooth sprocket/HS 142f. He pulled off the line with reasonable speed, and went right up to 41 mph.
With this information, and splitting the difference between 56 and 72 the difference is 64 teeth. Now were left with personal preference, rider weight, saddlebags, and preferred cruising speed, "AND" Speed Laws for this type of vehicle. Where I live it's 30 mph. I weigh around 185 lbs, and have saddle bags, so a 64 would probably be best, but I still want the bike's taxable horse power in the 25 mph to 30 mph range, so if the 64 tooth's top speed is 41, or more, to me anything below 62 is useless.
I just realized I could have ran across the busy intersection faster than the 56 tooth got me across. I would have to say I felt like a road hazard to other vehicles. Also it seemed like I spent more time listening to the engine lug than enjoying the ride. The engine actually started knocking after one big hill.
Keep in mind that a lot of the videos and post are 4 or more years old.