So, the tire temp display in LFS, is that the surface or carcass temp? Surface temps seem to vary greatly, much more than the LFS temps.

http://www.sportschau.de/sp/la ... x.jsp?id=139896#mbContent

That was already posted in some lengthy tyre discussion and in at least 2 or 3 other threads. Keep in mind that it was pretty cold on that day so the temperature variations are probably a bit exaggerated.

http://www.lfsforum.net/showthread.php?p=1676606#post1676606

LFS's tire temp display shows carcass temperature, unless you hold Ctrl-Shift, at which point it will show surface temperature until you release the keys.

Yet carcass temp is one that calculates grip.
Wouldn't it make more sense that surface temp is one used for it - as it's surface of rubber that has contact with tarmac?

Too much variation in temperature on the surface, depending on what your runnin gover. You want to know if your setup is good and how that is effecting the tyre temp overall, that means going into the carcass to give you an 'overview' of how the tyre is performing.

You missed my point.
LFS calculates grip based on carcass temp, while ignores surface temp.
However, rubber at surface temp makes contact - so that one should be used too.