Just getting back from an overseas trip. Here's an example of the blurriness I'm referring to other than when the car is completely stopped (even then not as focused so will need to work on that).
OK a lot going on there and I don't think you provided yet how zoomed in it is, but the closer you get to max zoom, the smaller the focus area is.
The blurry when the car first enters is due to being off-center of the infrared. Infrared works be reflecting back, so when it off the center, it has trouble reflecting back. That and that isn't the focus point I suspect.
Now the plate starts to get readable as it gets more centered and closer.
This is probably the cleanest shot.
I suggest you have someone go out and stop at say those three locations and then focus and you will see the focus number will be different for each one.
You then need to decide which one is the one you want to get the clean focus or average the focus numbers of the 3 spots and then recognize it won't be clean and crisp everywhere, but may be readable from more distances.
Even with my tight image where a car is in and out of the frame in under 0.5 seconds, as it gets close to and outside the red box, it gets blurry. The focus numbers starting top right corner to center to bottom left corner were different by 10 steps. So I went with the clear focus step in the center and can still read the plates outside of that, but they will be a little blurry.