Engaged couple Miley Cyrus and Liam Hemsworth are living life in the fast lane ... by riding around L.A. on Thursday in a $200K McLaren sports car.

While being her personal chauffeur may just be part of the job description, Liam took the driver's seat as Miley and her blonde 'do rode shotgun in the British high performance vehicle.

Miley and Liam's relationship really is going full speed ahead.