Insert is for when you have a scene within a scene, like a party where at one end of the room something is happening while the rest of the party is at the other end.
CLOSE ON: is for when you want to show something close up.
I only use it when I really NEED to show info that can't be shown any other way, or in preparation of a match cut to another scene. Again I emphasize to use sparingly. If I have 2 instances in an entire script, I'm pushing it.
Personally, showing a close up of a speedometer is not needed, especially since the audience can see that the car is going really fast without it.
Unless it is like speed, where if the car hit's 81mph it will blow up, or a ticking time bomb, etc...
Then again, it really needs to add to the story and not feel like filler, which from a few posts back, you seem to have indicated that you do. Fill with story, not filler.
Many new writers have the need to hit that 120 page mark and they always overwrite, way overwrite... It is one of the top signs of a newbie, next to camera directions, bad form, passive voice, etc...
120 is quickly turning to 115 or even 110 in the industry. In fact 115 is already there, just not in many how to books yet. I, as would anyone, would rather read a 105 script that is tight and fast paced and a quick read that to read 120 pages stuffed with filler.