I can't tell the exact benefit, because I have no fuel consumption display, but I experienced that it helps a bit (on a motorcycle it does, at least). Even from the safe 2+ second distance.
It's not worth it to get closer, though. Just like MMUK wrote, it's not "only" dangerous, you'll begin to follow the changes in the truck's speed with braking, so you'll end up losing what you had just gained.
Even safe distance drafting can have this effect under unfavorable conditions, so I don't recommend it except for motorways and other detail-less main roads. Otherwise it's better to be independent