Dario Franchitti kept Ganassi in front in the final free practice session ahead of IndyCar Series qualifying at Iowa Speedway.
After his team-mate Scott Dixon had led the morning session, Franchitti moved to the front of the field this afternoon with a time of 17.6856 seconds.
Franchitti's late lap usurped Penske driver Ryan Briscoe, who had missed much of the morning with fuel pump problems but recovered to set the pace in the first half of the split group session having studied team-mate Helio Castroneves' data from earlier.
Castroneves briefly held third in the closing minutes before Andretti Green's Tony Kanaan reclaimed the spot.
Dan Wheldon led the first group for a while for Panther Racing but was eventually pushed back to fifth overall, ahead of Dixon and KV Racing's Mario Moraes.
Behind eighth-placed Danica Patrick (AGR), Tomas Scheckter continued Dreyer & Reinbold's strong day with another ninth place. Graham Rahal completed the top ten for Newman/Haas/Lanigan.
Only 19 cars took part in the session as AGR worked to ready Marco Andretti's back-up car following the heavy accident that put his primary chassis out of commission this morning.
Pos Driver Team Time Gap 1. Dario Franchitti Ganassi 17.6805s 2. Ryan Briscoe Penske 17.6896s + 0.0091s 3. Tony Kanaan Andretti Green 17.6928s + 0.0123s 4. Helio Castroneves Penske 17.6953s + 0.0148s 5. Dan Wheldon Panther 17.7616s + 0.0811s 6. Scott Dixon Ganassi 17.7770s + 0.0965s 7. Mario Moraes KV 17.7802s + 0.0997s 8. Danica Patrick Andretti Green 17.8206s + 0.1401s 9. Tomas Scheckter Dreyer & Reinbold 17.8387s + 0.1582s 10. Graham Rahal Newman/Haas/Lanigan 17.8465s + 0.1660s 11. EJ Viso HVM 17.8611s + 0.1806s 12. Hideki Mutoh Andretti Green 17.8713s + 0.1908s 13. Ryan Hunter-Reay Foyt 17.9699s + 0.2894s 14. Justin Wilson Coyne 17.9704s + 0.2899s 15. Ed Carpenter Vision 17.9706s + 0.2901s 16. Mike Conway Dreyer & Reinbold 17.9802s + 0.2997s 17. Robert Doornbos Newman/Haas/Lanigan 18.0450s + 0.3645s 18. Raphael Matos Luczo Dragon 18.0581s + 0.3776s 19. Jaques Lazier 3G 18.5106s + 0.8301s 20. Marco Andretti Andretti Green