I would not exceed the 55 mph I were you.

I had a blow-out on my own car earlier in the year and had to drive on the spacesaver - not something I would recommend to do for any longer than absolutely necessary!

If I were you, I'd call Hertz to see if they will stand the cost (unlikely) and then get a replacement tyre PDQ. if you cannot (eg it's an awkward size and none are in stock), then I'd look to change the car.