Magnussen, who made his IndyCar debut at Mid-Ohio last season as a late substitute for an injured Felix Rosenqvist at Arrow McLaren SP, completed 143 laps in Alex Palou’s 2021 title-winning #10 machine.
The Dane, who will return to Ganassi’s IMSA SportsCar Championship lineup for the Sebring 12 Hours before taking up his Peugeot Le Mans Hypercar commitments, was the slowest of the 17 cars attending the first of the two days testing.
He whittled down to a 52.84s effort, which was fractionally under a second off Herta’s front-running pace, although the final classification is heavily caveated by the fact that some drivers tried push-to-pass boost while others did not.
Fresh from his LMP2 class victory in last month’s Daytona 24 Hours, Herta pipped Dale Coyne Racing rookie David Malukas to the top spot with a time of 51.85s that was just 0.01s faster than the Indy Lights graduate.
Malukas, who finished second in the Lights standings behind Kyle Kirkwood last season, took a full second off his morning time as the track rubbered in.
He will be joined on track for the second day of testing by former Rahal Letterman Lanigan driver Takuma Sato, as the two-time Indianapolis 500 readies for his first outing with the Coyne team.
Herta’s Andretti team-mate Romain Grosjean was 0.17s off top spot in third, the former Coyne driver having set the fastest time in the morning but setting a near identical time in the afternoon when others improved.
Team Penske drivers Josef Newgarden and Will Power were fourth and fifth, while Graham Rahal led the RLL contingent in sixth.
New Andretti Autosport signing Grosjean was third-fastest
Photo by: IndyCar
They were followed by Rosenqvist, who turned more laps this afternoon than any other driver to log the seventh fastest time, Alexander Rossi (Andretti) and Scott McLaughlin (Penske).
Rahal’s two new team-mates Jack Harvey and rookie Christian Lundgaard were separated by a mere hundredth, placing tenth and eleventh.
Patricio O’Ward could not replicate his morning time and his AMSP entry slipped down to 14th, behind rookies Kirkwood (Foyt Enterprises) and Devlin de Francesco in another Andretti entry.