The amount of thrust produced by an engine decreases with speed.
Takeoff performance is based on a specific amount of thrust, if this thrust isnt set by 80 kts, then it isnt achievable and you are not guaranteed your takeoff performance.
For example.
Mach / .00 / .10 /.20 / .30 / .40 / .50
Deg F / Thrust (lbs)
29.0 / 93169 / 83629 / 75589 / 69114 / 63248 / 54866
59.0 / 92861 / 83339 / 75305 / 68860 / 63006 / 54607
Draculas Teabag,
Your automation is a result of the above, the A/T system has nothing to do with the requirement to set takeoff thrust by 80 kts, it just helps you to do it.
Mutt.