I would leave it as is and when you are driving just do the math quickly in your head.
That's what I have to do in reverse to you, because our speed limit signs are in KM/H.
Quickly take the speed on the speedometer and multiply it by 0.6, and that will give you the rough figure.
So 100 KM/H is roughly 60 MP/H. 60 is 40, and so on.
Or you can use a piece of masking tape and write the conversion number on it, and stick it to the plastic lens on the right place with an arrow cut into the tape...