I have a 49cc that will do about 35 and a 52cc that will do about 30. Both to me are too slow for big city traffic. I built them for fun, but can only ride them only on backstreets. Very limiting. It's just not safe to ride them on city streets where most cars are doing 50-60 all the time in 30-45 speed limits here.
I do ride my 1400cc motorcycle that will do over 200mph, but was missing some fun with a homebuilt motorized bicycle. The slow result of the 49-52cc motors has me yearning also for a 125 or so dirtbike engine and trans for a bicycle. 40-60mph desired.
There's nothing safe about that or my motorcycle or the other two 30 mph bikes. That's not the purpose. I want to have more available power. Like my motorcycle, I have avoided being destroyed by a 3000 pound missile coming at me, from having 200hp to avoid them. That may not always work, but it helps.