I have now included tyre temperature effects. Tyre temperature
increases with rolling speed, skidding speed, proportionally to the
contact force with the road and decreases proportionally to the
airflow times air pressure (->airspeed squared).

Tyres now have an ideal operating temperature of 75oC, which is
implemented as a bell curve centered at 75oC and falling to 50%
efficiency at -25oC and 175oC.

Tyre wear is implemented as a rate of wear proportional to
skid amount and road contact force, while it depends exponentially on
the temperature (the rubber melts and starts to lose cohesiveness). It
is reduce for stiffer tyres. (Tyres with high K, IIRC). Tyre width is
also taken into account (since there is more material to strip away).

Currently the tyre performance is proportional to 1-tyre_wear,
although that should probably change so that the first part of wearing
out the tyre should not have any effect on its performance.