It can be really simple for the keypress checking.
Add an instance variable for the max speed and do something like:
when Throttle key is pressed condition on the same block with a maxSpeed>100 . Then the action set to add 1 to the max speed variable.
That is so that 100 is the cap, and they can not hold the throttle key longer. then set a decelerate key to subtract from the maxspeed variable or have it reset on a throttle key up condition.
It would probably take less processing power to just cap the throttle rather than trying to get the current speed of something, and then capping it.
I can see when getting the current speed would be needed, especially if the car collides with things that will slow it down.
The above example would be better with something like a space ship, but with a little complexity you can probably make it work for you.