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I use several reference to aid me
First I drove the car on CH to CS, then look at the G meter under cornering loads ( low to high speed ), then if there's lap time data available, I use them as benchmark. I drove on conservative pace, meaning not killing the tires for ultimate laps
Then look how faster or slower the tires are to real life lap time on average. Then I also tested acceleration grip from standing start, see if the tire that I got close to has adequate grip not just on lateral loads. I always make sure I got the car weight correct first, lighter cars can affect how the tire can hold a line at speed.
These should be enough to get pretty close to what kind of tires were fitted from factory back in the day the car was sold. Like the Countach for example, fitting CS on these are a bit overkill, but of course, one can just imagine if these Countach were to be fitted with modern radial track ready tires
CS or SH would be perfect.
The 512BB and Countach were best on CH when measured against real life acceleration times from 0 - 160kmh, 0-400m etc. The Yellowbird works best on CM or CH if wanted more raw handling closer to Assetto Corsa Yellowbird lowest grip tires ( lewis_hamilton said about this on his review vs Assetto Corsa YB )
People often forgot or didn't realize how much speed they are driving the cars around a corner in GT6, often they feel slow, but try do similar speed in real life, I can guarantee even medium performance summer tires would squeal a lot at those speeds