How fast depends on how long you dared to hang onto a passing truck.
When I was younger, most bikes were single speed cruiser type bikes, it was common place to grab hold of the hand bar on the back of a passing truck for a quick tow.
Certainly not safe but I'm still here. More than once I remember thinking that stamped steel bike hubs with loose bearings probably weren't meant to go that fast.
These days my problem is stopping, as i get older, and heavier, a few local hills here, (and they aren't very big hills), tend to turn coaster brake hubs into a grease bubbling mess.
If my phone GPS is right, I hit about 23mph on the one hill near my house here. It don't seem like much of a hill till you try and stop a 70lb bike with a 350 lb rider on it.