The 30-year-old Englishman, who first drove for Panther in two races six years ago, will also test at Sebring, Fla., before heading to Australia for the Oct. 26 IRL race at Surfers Paradise.
"It's a great opportunity for us to work together on a race weekend and determine what the team needs from me and what I need from them," Wheldon said Tuesday.
"The key to the 2009 championship is coming out strong in the first five races, and the work we do now will help us accomplish that. In just the first few days I've spent with the team I must say I'm impressed with how hard the guys at Panther work and the attention to detail I've seen with the race cars."
Wheldon joined the IRL as a test driver for Panther in 2002 and made his first start late that season at Chicagoland Speedway, where he finished 10th. He won the Indianapolis 500 and the IndyCar championship with Andretti Green Racing in 2005 and drove the past three years for Ganassi Racing.
His spot with Ganassi will be filled next season by the 2007 Indy 500 and IndyCar champion Dario Franchitti, who is returning to the IRL after struggling in his one season in NASCAR.
Ganassi's Scott Dixon won at Indianapolis this year, one of a record-tying six wins, and claimed the series championship by 17 points over Helio Castroneves, who beat him in the season finale at Chicagoland by one foot in a photo finish.
Wheldon will replace Brazil's Vitor Meira with Panther.
"We look at this as a great way for our team to get a jump on the 2009 season with Dan," team co-owner John Barnes said. "Our goals for next season are high, and the work we're putting in to meet our goals of winning races, the league championship and the Indy 500 have already begun."
Franchitti headed back to IndyCar Series
We Have The Strength In Depth - Pires
