It seems that iRacing think their new tyre physics will be ready this summer.
I don't know how many of their team have been working on the physics but they admit it was the hardest part to do.
Here's a snippet from their press release.
In a major press release today, iRacing outlines its ambitious summer update. At the heart of the 2.0 summer-long update is the new tire model, which the release says “will revolutionize the way iRacing members race online, providing unparalleled realism and the most accurate racing experience ever produced for a PC.” iRacing’s CEO Dave Kaemmer said, “This new tire model is our best work ever. It was incredibly challenging and time consuming to develop an understanding, mathematically speaking, of a racing tire at the limit and under all sorts of conditions… The tires are clearly the hardest thing to simulate for any racing title, but I think we’ve nailed it! I believe it is going to bring simulated racing to a whole new level of enjoyment and realism.”