You can safely use muriatic acid on rubber but please be very careful. This is very dangerous stuff that can do significant damage to the rest of your car...and yourself!
Even the fumes from this substance can do major damage, so please be sure to wear a mask, goggles and rubber gloves.
Also be very careful with your car, it will obviously rust paint but it also doesn't like aluminium either and will eventually eat into it.
Muriatic acid will strip concrete too, so be sure to use a tarpaulin in the area you will be working, or cover the area in water to dilute some of the acid.
It can be used in diluted form to clean the wheels on your car too, but rinse it straight off, or it will begin to eat into your wheels.
The acid needs to be safely stored and safely disposed of, so please make sure you are clear on how to do this before you begin to use it.