Loeb's time was 0.146 seconds faster than Evgeny Novikov, the Russian star running DMACK tyres throughout the final round of this year's World Rally Championship.
Loeb's team-mate Mikko Hirvonen was third, with top factory Ford driver Jari-Matti Latvala fourth. Latvala's team-mate Petter Solberg was one place further back in fifth with Thierry Neuville (Citroen) sixth.
Local hero Dani Sordo's Mini was seventh, 1.384 seconds off Loeb's benchmark.
The qualifying winner - and winner of this event for the last seven years - said: "I was not pushing very, very hard to make the fastest time this morning, but in the end it has happened.
"I was thinking this morning that it wouldn't make such a difference if I was the first, second or third or fourth to pick my place [on the road].
"When I think to the first day and where I will run on the road, we have to remember that even if the rain doesn't come, the road this morning was damp. It's not the easy choice."
If damp conditions prevail tomorrow morning, first on the road could be the fastest position, as the cleaning effect of cars sweeping loose stones from the surface would not be as pronounced.
Loeb added that his considerations for the event went well beyond tomorrow's gravel stages.
"We have to think about the asphalt as well," he said. "This is not straightforward: Jari-Matti [Latvala] and Mikko [Hirvonen] both have very strong pace for this rally and we have to be wary of them."