Chunked tires

[Dave Priebe]

After the last drivers event at Pacific Raceways in March, my passenger
side tire is pretty 'chunked' up. How can I tell when too much
'chunking' means its time for a new set of tires? The car still seems to
drive just fine, but that tire looks terrible. There still is a good bit
of tread (~25%) remaining, and I would like to have these tires last for
as long as possible (at least through the next event at the end of May).
Should I be concerned that it may be a safety issue?

Thanks,
Dave Priebe
Kenmore, WA
91 200tqa 169k his
95.5 S6 104k hers