Actually on my 2000 tomos all I needed to hit 70kph (roughly 43mph on level ground was the biturbo (aside from that its straight stock 50cc, no 70cc kit at all) It's quite good about getting up to 60-65 then takes me a bit to get up to 75.. which is about the fastest I've ever had it.
As someone else said, the brakes aren't really up to snuff for those speeds and you need to allow extra time for braking, plus the lighter weight of the mopeds can cause a bit more wind buffeting - especially at higher speeds. On average I ride mine at 40-50kph which is fine for city driving.