On the day the split officially ended, Dan Wheldon led the way during a six-hour evening test session at Homestead-Miami Speedway.
The Target Chip Ganassi Racing driver set a lap of 25.0619 seconds (213.312mph) in the No.10 Dallara-Honda.
"The announcement today was great, but there's not anything significant in (going fast on the same day of the announcement)," said Wheldon, who won last year's opener at Homestead.
"When you get down to work, you have to make the most of it. If I win the race, I might feel something special, but not now."
The test was the first of the 2008 season and the first since the end of the 12-year split between open-wheel's sanctioning bodies. IndyCar Series founder Tony George and former Champ Car co-owner Kevin Kalkhoven announced earlier in the day that Champ Car teams will join the IndyCar Series later this season, perhaps as early as the season opener at Homestead on March 29.
No Champ Car teams participated in Wednesday's test, nor are they expected to participate in another six-hour night session on Thursday or in a three-day test next week at Sebring International Raceway.
However, as many as nine teams could take part in additional tests for former Champ Car teams next month at Homestead and Sebring. Dates of those tests have not been announced.
Wheldon was just slightly faster than his teammate, Scott Dixon, who was followed by Penske's Helio Castroneves, Andretti-Green Racing's Tony Kanaan and Penske's Ryan Briscoe.
Dixon, last year's IndyCar Series runner-up, also said he felt no significance in being among the fastest of the usual frontrunners.
"Nothing has changed at the moment," Dixon said. "Not until everybody is out there."
Testing resumes Thursday at 4pm Eastern Time.
Wednesday's testing times:
Pos Driver Team Time
1. Dan Wheldon Ganassi 25.0619
2. Scott Dixon Ganassi 25.0660
3. Helio Castroneves Penske 25.1211
4. Tony Kanaan Andretti-Green 25.1227
5. Ryan Briscoe Penske 25.2127
6. AJ Foyt IV Vision 25.2179
7. Marco Andretti Andretti-Green 25.2465
8. Hideki Mutoh Andretti-Green 25.2476
9. Ed Carpenter Vision 25.2691
10. Buddy Rice Dreyer & Reinbold 25.3716
11. Ryan Hunter-Reay Rahal-Letterman 25.3934
12. Vitor Meira Panther 25.3987
13. Milka Duno Dreyer & Reinbold 25.6276
14. Darren Manning Foyt 25.6369