I think the dry grip on asphalt on most tracks is set at 1.00 unless there's something special going on. What does this number of 1.00 represent in the real world? At first I thought it was an indication of the road friction coefficient, μ, but the new asphalt in the TDF is set at 1.02 and above 1.00. For example, it is often said that the pavement at the international circuit in Japan, where I live, has more grip than the pavement at other countries' circuits. On the other hand, the pavement I made for Sendai Hiland Raceway was not a special asphalt pavement for the circuit, but the asphalt pavement used for normal roads. With this in mind, I set the dry grip value of asphalt in the Sendai Hiland TDF file to 0.97. Personally, I think the grip is a little too little. If there was no particular standard, and the dry grip on most circuits was 1.00, then Especially good circuit paving > "most" circuit paving = 1.00 > "normal" asphalt paving or public roads Is this the right way to think? If you have a good TDF value for each type of road surface, please let us know.
I think you're on the right track (!!! Pun intended! ) 1.0 is just a nominal level of grip considered "normal" in the game. Tyre grip and performance is then built on that, so a surface with better than normal grip should be >1.0, while a slippery surface should be <1.0. 0.97 doesn't seem far below, but especially under power it can make a substantial difference in feel. So I wouldn't stray too far from the standard value, certainly for the whole track.
From my real-life autox experience and datalogging using SoloStorm, I can say 2-3% difference is about where I start to notice differences in grip for cornering.
Laptimes laptimes laptimes. They should be quite good suggestion for surface grip. Another important thing to consider is wet surface grip. The way I understand general rule should be that civil type of tarmac could be expected to be rougher, thus less adhesion but more mechanical keying, which is beneficial when wet. Unfortunately I find roughness parameters not to be working correctly right now in rf2. I expect S397 to tackle some physics in 2021.
TDF value for Sendai Hilands. There is a website that allows you to guess the times of other circuits based on the times of Japanese circuits. The accuracy of that website is debatable, but I'll believe this for now As a result of the comparison as, the number settles to 0.99.