Three teams and five drivers concluded their two-day test session at Iowa Speedway on Thursday.
Danica Patrick and Marco Andretti joined Ganassi drivers Scott Dixon and Dan Wheldon and Penske's Helio Castroneves in a private test designed to find a more suitable aerodynamic package for next year's race at the track.
"We thought it was a short track, but it's really a superspeedway," Andretti said. "We were as easily flat-out here as we are at Michigan or Chicago. We thought it was going to be Richmond, but it wasn't. That's what we're trying to achieve now."
All five cars were on the track simultaneously through most of Thursday's session. No data were released, but drivers indicated they were closing in on an aero sweet spot that will allow the cars to use more than one line on the 0.875-mile oval.
"We're working on setups that will allow us to pass each other," Patrick said. "We need to be able to run close together, and they've been taking away downforce. That causes the cars to not be able to run close together, so from that standpoint it defeats the purpose.
"We've been debriefing with all the teams together and trying to find something that works."
Eight of the 19 cars crashed out of the inaugural IndyCar race at Iowa Speedway in June. Dario Franchitti won over Andretti in a race that turned into a single-file parade.
By experimenting with different wing configurations during the test, IRL and team officials hope to create conditions suitable for side-by-side racing and sufficient passing.
"It's not that easy," Andretti said. "It's a very fine line. If you take downforce away, you can't get close to another car. The opposite of that is if you have too much downforce so we can just sit there, easily flat out, but we can't pass each other.
"That's probably what we had (in June). We want to keep it close, but we want to be able to pass. That's very challenging."
The series will hold an open test October 12 at Barber Motorsports Park near Birmingham, Alabama.