Ask Ralf Schumacher or one of the Michelin F1 tire guys about running an under-spec'd tire...






or




How the tire would actually fail is rather academic, since if it did you'd have a whole lot of other things to worry about than whether it was thermal or rotational stress that was the root cause of your car cartwheeling down the road. Just go by the code stampled into the sidewalls. Its not worth testing whether all those factory tire and chassis engineers were right or not, IMO.

But first, if you are running a mis-matched set low speed tires, you really should spend your energy on acquiring a decent set of V (or Z) rated tires, especially before trying to set any land-speed records. And please tell me your choice of rubber currently isn't those stock M+S all-season 14" RS doughnuts!

Yes, the national 55mph limit was a result of the 1970s oil crisis. In the mid 80s it went to 65, due to the fact that most people were ignoring it, and that fuel efficiency had increased a lot by then. In '95, the Feds let the states mandate their own limits. Nowadays, the biggest reason behind where speed limits are set is probably 'revenue generation' by local politicians and police forces...