How to tell when slicks need to be replaced?

[XLR8]

How do you know when it's time for new tires? There are no tread wear indicators. Do you just use them until you see cords? Would your 60' start to suffer? My Hoosier QTPs are on their 3rd season and I must have over 50 passes on them. There is a little feathering, but I swap the tires from right to left every trip to the track.

[blk00z28]

I thought most slicks had little dimples in them to show you where or when rather you would need new ones?

[tonka]

I've seen guys at the track with a depth probe of sorts checking the depth of the holes across the tread of the tire, and basing treadlife off of those depth readings.

[salvageV6]

My 27 x 10.5 Hoosier QTP's have little holes in them at various points around them.

I figure when the holes disappear I only have about 50 passes left on them.

Not sure how long those 3 little grooves are gonna last either.

[XLR8]

I'll keep an eye on the holes - the groves are already gone.