Wiktionary
n. A car that other cars drive behind before a race starts.
WordNet
n. a high-performance car that leads a parade of competing cars through the pace lap and then pulls off the course
Usage examples of "pace car".
It sounds to Henry like the Indy 500 just before the pace car runs for dear life and the checkered flag drops.
The Chevy just kept coming at a steady fifteen or so, like the pace car in the Tournament of Roses parade.
Hooter moaned louder as the minivan crept ahead in the barely moving gridlock of race fans, all of whom were getting out of sorts and worrying about missing the start of the race, when the pace car would roar out onto the track and Air Force F-16s would fly over in formation.
The car passed and continued on down Kansas Street, as slow and serene as the pace car in the Tournament of Roses Parade.