Does Mixing different Speed Tires really matter that much!
 

I would think that would be ok...as long as the speed ratings weren't too far apart.
And I would put the highest rated tires on the front, just in case.

DON'T mix them on the same axle.

They will probably have different slip ratios, so handling might be strange if you have different ones front vs. rear. But that's a bit more subtle and only if you go fast.

This belongs in the tire area, so I'll move it.