The Reasoning that I have for deeming a photo realistic, is not based on the fact that the car model looks like the real car... its actually everything besides the car model... its the lighting, the reflections, motion blur etc.
Rule number one for having a realistic picture... have as few FAKE looking things in the picture as possible... if Necessary, Blur the whole picture except for the most realistic part of the picture.
Find a focus point, If you are trying to accentuate realism, make the focus on something tangible... 99% of Vehicle owners, spend more time INSIDE the car than outside... When you search for details on the outside of the car, your mind goes crazy, looking for patterns, so that you can find your bearings.
Taking this into your mind, The outside of your car reflects light, which in turn create patterns... the more realistic this pattern is, the more realistic your photo will be.
Going even further into this thinking, having FIXED patterns on a car, (ie Race Livery) makes the car MORE of the focus of the photo, and provided the Livery looks realistic, and Non-Pixelated, you are one step closer to realism.
Having things like props, which look great while you are traveling on the track at 100+, tend to lose their charm when motionless... The cars are made up of MILLIONS of Polygons. a single blade of grass is made out of 1 polygon... its bright green, casts no shadow, and will completely ruin an otherwise decent shot.
Another prop to avoid... people... bystanders, pitcrew, even the drivers... they easily make your pictures look like LEGO in a $400 1:50 scale model display... avoid them when stationary, and at a high rate of speed, a little motion blur will make them look the way they would look from inside the car... blurry.
More tips later.