Learn Turkish: Arabanın hızı çok yüksek. - The car's speed is very high.

This is a specific noun compound showing possession. Because you are talking about the speed of a specific car (the car's speed), you need the genitive suffix -nın on araba and the possessive suffix -ı on hız. Araba hızı would just refer to the general concept of car speed.

Why use yüksek (high) here instead of hızlı (fast)?

It matches how we talk about measurements. While a car itself can be fast (araba hızlı), the speed of the car is a level or a number, so it is high (hız yüksek). Saying the speed itself is fast sounds unnatural in Turkish.

Where is the word for "is" in this sentence?

For simple statements in the present tense using "he/she/it", Turkish doesn't require a visible "to be" verb. The adjective yüksek sits at the end of the sentence and acts as the whole predicate, so "is high" is naturally understood.

How would I use kadar with this sentence to say "The car's speed is as high as a plane"?

You place the thing you are comparing it to right before kadar, and put both of them before the adjective. You would say: Arabanın hızı uçak kadar yüksek.