Depends upon track:
15x42t tighter tracks Sonoma, Thunderhill West, Laguna Seca, Streets of Willow Springs
15x41t bigger tracks Thunderhill East, Buttonwillow, Willow Springs
Although gearing for top-speed on straights may not be fastest. Sometimes, gearing for best drive out of corners may be better and to skip a shift. Also account for getting tow down longest straights and getting drive out next corner from defensive line may be best.