Range of Movement (or Speed) is a hidden attribute in XCOM EU.
A soldier's range of movement value is 12, which translates into 7 cells for a half-turn move, and 15 cells when dashing - both when running a straight line.
According to Arqade, the conversion for a half-turn move is:
round_down(attribute value * 0.625)
So, with a value of 12, we get: 12 * 0.625 = 7.5 => round_down(7.5) = 7
With the Second Wave option Not Created Equally, the movement attribute may vary between 11 (6 and 13 cells) and 14 (8 and 17 cells).
With the option Hidden Potential the movement attribute may increase when a Soldier is promoted. This doesn't seem to be the case, when Hidden Potential is not selected.