180/55 is prefect for the street. If you find that you cannnot get a 180/55 tire that will give you enough traction for the street, you really need to take it to the track.
I actually always ran 180/55 on the track as well, but a 190/55 would be a better choice.
A 190/50 is the worst choice of all.
Here's a visual aid for those wondering...
