Indianapolis 500
Technical regulations

Teams may enter up to two machines under a given car number—the "primary" car and a "backup" car. The backup car is identified by the letter "T". For example, the two cars for the #2 team would be numbered #2 and #2T. Both cars may be practiced during the month, but due to engine lease rules, they must share the same engine. It is not uncommon for teams to prefer their backup car, if it is deemed faster, or for other strategic reasons. Additionally, as the month wears on, a "T car" may be split off into a separate entry, and reassigned a new number, or be sold to another team.
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