Here's mythbusters doing it: https://www.youtube.com/watch?v=lttgT1XZVvE

They estimate the savings at insane 20 feet at 20 something. Far from your claimed 100%.

Could it be, that your road is not as flat as you think? That would also explain the necessary braking to not run into the truck, wouldn't it?

But I couldn't agree more: let the machines do that perfectly and safe. Let's reap those sweet 25-50% increase in fuel efficiency.