It's a question of what level of accuracy you need / want / care about. Whether +/- 10 mph is acceptable, or +/- 5, or +/- 1, or +/- 0.1.

Let's take the most common approach for most people's cars, and what shows up on your speedometer. You know engine speed, you know transmission ratios and final drive, so you know rotational speed at the wheels.

But it's not a true measurement of ground speed.

You don't know how much wheel slip you have (which can be significant in a high power racecar), nor do you know the dynamic state of the tire and its rolling radius. At 200 mph you can easily be off 5-10 mph.

Then there's GPS, and there's a slew of options there with regard to fidelity - from inexpensive to quite expensive. The easy example of this is if you put a GPS device in a single spot on your kitchen counter and let it log while stationary - it will tell you that it's floating around when in reality it isn't.

Granted, relative position / velocity is better than absolute, but you need quite the refresh rate to something in space while you're whipping around at speed. There are "local" GPS systems but they are quite expensive.

And why bother with it? To roughly show where a car is on track and get a rough gauge of how fast it's going - that's good enough.