Those mini spares have sizes, speed and load ratings like normal tires do. Mine has a speed rating of M or 130 KmH//80 MPH.
That said, use common sense. These tires don't have anywhere near the grip of normal ones. That's why they say not to exceed 50 or whatever MPH. It has to do with breaking and handling. If at all possible, put it on the rear even if a front tire went flat. The longer you need to have it on the car, the truer this advice becomes. I drove around for a week on one, nearly 800 miles at motorway speeds.
Here's the mounting sequence for a flat front tire when you know you can't replace or repair the flat right away...
Park car. Apply hand brake, turn on four way flashers, put on safety vest and set out warning triangle. Put the spare on the left rear (for RHD cars) of the car. Take the good tire and use it to replace the flat. Put the flat in the trunk//hatch//boot//wherever.
So why take the left rear tire? That's important if you are at the side of the road and have to change it in place. It will cut in half your exposure to traffic while changing. Of course if there's no hard shoulder on the left, use the right.