I wanted to do a post about this because I see so many people asking on different forums about why their footage is blurry and most people just say to "increase FPS", or "use a higher FPS and that will fix everything". However, that isn't the only thing to consider. Plus, I've seen others that are already shooting at 60FPS or 120FPS and the slo mo still looks blurry or like they've forgotten to clear the vaseline off the lense? What gives?
Here's the TL;DR version: Use at least double the number of your frame rate for the shutter speed, eg for 60FPS have your shutter at 1/120 if you can. For 120FPS have your shutter at 1/240 (or as close as possilbe, often 1/250) or even faster speed for clearer footage.
You need to understand the key difference between FPS and shutter speed, but also why they are both important, especially for videoing small cars travelling at high speed.
FPS or Frames Per Second is important because this is basically how many frames a car will spend in your shot - this is vital when you have a finish line cam as some cars will cover a large distance in milliseconds. Let's presume you have a locked off camera, just means set in place rather than following the car, for your finish line area and it's shooting at 60FPS. If you car takes a quarter of a second to cover from one edge of frame to the other you will have 15 frames with your car in it (60FPS x .25). If you are shooting at 24FPS then you get 6 frames and if you're shooting at 120FPS then you'd have 30 frames. How often have you had to go frame by frame to see who won a race only to find 1 frame they are a couple inches away from the finish line, then the next frame they are on or past the finish line? That's frame rate right there, whether it's blurry or sharp.
Great, so higher frame rate means more shots of the car, problem solved. Well actually no, this is where your shutter speed comes in. If you have manual control of shutter (like on an action camera and some cellphone cameras) and you have left it at the standard rate there is a good chance that your 120FPS footage will look like garbage because your cars are totally distorted and blurry. This is because the shutter speed determines how long the "film" is exposed to the subject - the shorter the time, the sharper the resulting image as the object hasn't been able to move as far in the time the shutter was open, as it did for a slower shutter speed.
There is a film making "rule" out there, called the 180 degree rule for shutter speed, it basically means if you shoot at 60FPS, your shutter should be 1/120. This is primarily aimed at cinematography as it gives a more natural blur to things, however when you're slowing your footage down to see who won the race you need a clear and sharp image so you can break the rule too. Here it is better to go for a higher shutter speed if possible. So if you shoot at 60FPS and you can set your shutter to 1/240 then the image should be a lot sharper again when slowed down or going frame by frame. Just keep in mind that this also means you need better lighting at your track because less light makes it into the camera, making your footage appear darker.
Here's a video I found that explains it in more detail (still only about 9mins of your life) and has some really clear examples of different shutter speeds and frame rates and how they make your video look.
FYI - This works exactly the same in photography - fast shutter speeds freeze action, slow speeds show blur.
Hope this helps solve some issues for people and we can all improve the footage that we get :)