The speed something has is the distance it travels in a certain amount of time. You can work out a car’s average speed by dividing the total distance it travels by the total time it takes.
The acceleration something has is the change in speed over a period of time. A cars average acceleration is measured in different ways, sometimes they just time it getting from 0-60 as fast as possible.
Scientists often use the word velocity instead of speed, dont get confused, this isn't quite the same. Velocity is your speed in a certain direction, so if you drive north at 50mph for an hour, then south at 50mph for an hour, your average speed is 50mph, but your average velocity? Zero! This is something you should think about.
When skydivers leap out of planes they wait a whole minute to deploy their parachute. With nothing to stop the gravity, yanking them out of the sky, they accelerate like rocks, but after a few secconds, they hit a magical speed.
Around 125 mph, is what scientists call, "terminal velocity." so why does this happen?
This happens because gravity isn't the only force acting on them, there is an increasingly powerful force from the air rushing past. Drag. Eventually the drag balances out the gravity so the skydivers stop accelerating. The same thing happens with cars, the force of drag becomes as strong as the force of the engine so the car reaches top speed.
Speed is also affected by Air Flow.