Dunno what turn motor you used with Cirtix, but X12 17.5t and 13.5t motors worked just fine for me when I tried them before and won race with it. Now my dad uses it (already done 3 or 4 races with it) with above mentioned 17.5t and works a treat.
Maybe Cirtix speedoes aren't reliable with X12 motors in general, but mine works just fine.