To reach millions of viewers in a day, you need highly relevant and trending tags. Here are some car-related tags that are popular and have broad appeal:

1. #Cars
2. #CarLovers
3. #CarLife
4. #Auto
5. #CarCulture
6. #CarGoals
7. #Supercars
8. #LuxuryCars
9. #SportsCars
10. #CarAddicts
11. #Vehicle
12. #CarPhotography
13. #CarOfTheDay
14. #MotorTrend
15. #CarPorn
16. #AutoWorld
17. #FastCars
18. #ElectricCars
19. #CarVlog
20. #CarNews
21. #ModifiedCars
22. #ClassicCars
23. #CustomCars
24. #DreamCars
25. #CarDesign

Using a mix of popular, niche, and specific tags can help boost the visibility of your content. Make sure the tags align with the content you're sharing for the best results.