Leclerc's best time of 01:31.992 seconds was 0.879 quicker than McLaren's reigning champion Lando Norris, second on the timesheets, and fastest of the three-day second test.
"We ran through our programme and tested all the things we planned to," Leclerc said.
"In terms of performance, it’s still difficult to understand where we really stand because teams are hiding their true form, so it’s important not to focus too much on lap times and prepare for the first race."
Mercedes have suggested Red Bull are the benchmark, and pointed also to Ferrari's rapid starts, while Red Bull have positioned themselves further down the pecking order and identified Mercedes as being a step ahead. The reality should become clearer when the racing starts in Australia on March 8.
Four-time world champion Max Verstappen was third for Red Bull, 1.117 off the pace, and pre-season favourite George Russell was fourth for Mercedes and well off Kimi Antonelli's Thursday best of 1:32.803.
"I think it is quite obvious, of course, that there is still quite a bit of work to do for us to be faster so this is something we will be working on," said Verstappen.
Leclerc was in the car all day, with teammate and seven-times world champion Lewis Hamilton watching from the garage, and completed 135 laps of the Sakhir circuit.
Alpine, last overall in 2025, had Pierre Gasly fifth fastest with Oliver Bearman sixth for Haas and Gabriel Bortoleto seventh for the Audi works team that has replaced Sauber on the starting grid.
Lindblad, the sole rookie this season, was ninth and set a record for laps in a single day in Bahrain with the Racing Bulls car looking solid with the new Red Bull engine. The previous record was 161 by McLaren's Oscar Piastri last week.
At the other end of the table, troubled Aston Martin completed only six laps with Lance Stroll scheduled to be in the car all day but failing to set a time with the team sidelined by a battery problem.
"It’s clear the car isn’t where we want it to be performance-wise, and we know there’s a lot of work ahead in the coming weeks and months," said Stroll.