Just happened to stumble on these recently. The make my head hurt to watch them, but interesting nonetheless. Side by side comparisons of Real Racing 3 and real life. All done by Ray Gamer. They have lots and lots of RR3 videos, but these are the only side by sides I found.

The sounds are from the real world. Although the monkeys have improved the engine noises over the life if the game, they still have considerable room for improvement.

Porche 919 on Le Mans (I think RR3's Porsche needs its headlights adjusted. Something beyond the next 20 feet.)

McLaren P1 GTR on the Spa...

Daytona NASCAR

Ferraris on Panorama