Pandora Media Inc., the biggest online radio service, said the number of US listeners in cars topped 2.5 million, a milestone in efforts to win customers from traditional stations. The stock rose the most since March 8. Listeners obtain the service from 23 auto brands and eight car-stereo makers. This year, Pandora will be available in more than 100 models. About half of all listening takes place in autos, a crucial battleground in the $15 billion local radio advertising market.