I guess when your game looks that good, you're pretty much limited when it comes to comparisons. YouTuber ADRIANF1esp, to showcase the graphics of — you guessed it — Project CARS, took camera footage of a BMW doing a lap on the Laguna Seca Raceway, and put it next to an in-game run on the same track, using the same car.
The result is the video below:
The real-life footage appears to have been taken with a low-resolution camera, which is a shame, certainly. It kinda-sorta makes real life look worse in comparison. Still, it's a great watch! Spotting all the differences is fun, too.