It does not automatically make them at fault, but it is evidence that they were at fault. Speeding is often careless, or negligent, which can translate into fault, but you have to view it in the context of the situation. Someone going 60 in a 25 mph zone who rear-ends another car because he couldn't slow  or stop in time is very likely at fault; someone going 65 in a 55 mph zone who is hit from the side by another car which changed lanes without looking is not at fault (and the improper lane-changer would be at fault).