In theory if the insurance company could pin the blame on something like you fitting
a tyre that was only rated for 70mph and you were doing 158mph and the incident
was a direct cause then they could argue against paying out.
No mention of load ratings in my manual, but the same could apply assuming you could
get a Mondeo sized tyre with a sub 65 load rating (290kg x4 1160kg) and stick them
on an estate car loaded to 2200kg and drove above the speed limit and crashed
because of that they could proecute you and the insurance again could claim
you caused the incident.
Would your MOT tester spot a tyre rated below whats suitable for the car?