Color IndyCar teams' tire choices red and black
Each car in today's race gets six sets of primary tires and three sets of alternate tires, known as "reds" because of their red sidewalls. The primary tires are black tires.
The blacks are more durable but not quite as fast, especially in the first couple of laps. The reds are faster and have more grip but don't last as long, and once they wear they lose performance quickly. Both types of Firestone tires have all-new specifications this season.
All cars must use at least one set of each tire type for at least two green flag laps in the race. After Saturday's qualifying, teams had to declare which type of tire they would start today's race with. These rules apply on all street and road courses in IndyCar.
And if it rains? The red/black rule goes out the window and the cars will use grooved tires, which help the cars get through the water, rather than the grooveless "slicks" normally used. This is unlikely to come into play today as weather.com calls for only a 20-30 percent chance of rain.