Thanks. I wasn't sure if anyone else here would be familiar with the NIIRS scale. Assuming we're only talking about visible light here, (SAR and IR wouldn't read the colored letters on the tag) the standard for NIIRS 9 is to read an actual license plate number.
For those who are unfamiliar with NIIRS, 9 is the end of the scale. Most commercially available satellite imagery (google earth) is NIIRS 4 or 5.