Exactly. Similar to how others responded to a similar comment to yours, there isn't much slipstream savings without getting into the danger zone of around 30m (100ft), which you can't get with 3 second gaps at speed.

But you do get more fuel savings from driving at that slower speed, and from coasting and accelerating lightly/cruising. It perplexes me why people alternate gunning it with braking so much on the highway.