"Optimum" depends on what you want to optimize for!
Less risk? Further away.
Less wind resistance? Closer.
The greatest energy savings come when within 12" of the vehicle in front of you, but that's much to risky until we have automated computer control linking the vehicle train.
I've played with drafting a lot in my 2000 Honda Insight - the bigger the vehicle you're drafting, the more protection you get, even far away, but "big" is defined by the profile, not the length. I'll explain:
You get some benefit from drafting a regular car, but you must be close.
You get more benefit drafting an SUV or big pickup truck, even if you're slightly further back.
You get even more behind a box truck or bus. Bigger and bigger profiles.
You do NOT get even more behind many tractor-trailers <-- surprise!
Why no increase from the bigger rigs? Because they are very open on the bottom. There's a lot of airflow underneath them, and if you get close, you're driving into the headwind from under the trailer!
However, more and more trailers have wind baffles installed on the sides. If the trailer has the wind baffles on the sides, then the big rig will give you more protection.
As others have said, though, you run a risk from the tires kicking up stones and road debris from in front of you, and you also need to be ready to brake in case of accident, so this practice is NOT without risk.
Safe travels!
