Speed points do not directly correlate with squares per minute. Each type of tile has a set "friction" which requires a certain speed to overcome. These speeds are known as "breakpoints" and after reaching a certain speed the character will move faster over these tiles. The breakpoints can be seen here, but its possible they may have changed at some point in time.
One way to figure out your speed would be to set a stopwatch and run a set number of the same type tiles in a straight line eg 50 tiles. and extrapolate that information into tiles per minute.