The Google Adwords Keyword Tool External says these keywords have decent search volume.

[car race games]
Global Monthly Search Volume: 27100
CARRACEGAMES.ORG

[bike racing games]
Global Monthly Search Volume: 22200
BIKERACINGGAMES.NET
BIKERACINGGAMES.ORG

[dirt bike racing games]
Global Monthly Search Volume: 22200
DIRTBIKERACINGGAMES.NET
DIRTBIKERACINGGAMES.ORG

[online racing game]
Global Monthly Search Volume: 9900
ONLINERACINGGAME.ORG

[play racing games]
Global Monthly Search Volume: 9900
PLAYRACINGGAMES.ORG

[truck racing games]
Global Monthly Search Volume: 8100
TRUCKRACINGGAMES.NET
TRUCKRACINGGAMES.ORG