Not totally correct Chris.
When I was the Principal Engineer for the Australian Federal Transport Department we were certainly regularly lobbied by various parties of various genuine and/or vested interested parties to introduce National Legislation to limit tyre-life based on an arbitrary age, over and above the prevailing in-service Roadworthy Requirements such as tread-depth, visible damage etc.
I can’t remember all the detail of the argument and counter argument, but bottom line is there was no technical justification we could find for arbitrarily putting in place a specific maximum tyre-life age requirement on justifiable/demonstrable safety grounds, but rather than debate the technical merits or lack thereof, we chose the easy cop-out route than be subjected to law suits and class-action suits regardless of which way we went, and said it was not a Federal Legislation matter, as the ACT of Parliament, under which Federal Automotive Safety Laws applied, was only applicable to cars when they first entered Service for use on Australian Roads, so only for new tyres, thus any suggestion of a 10 year old (or other arbitrary number) restriction was not within the Feds area of interest/jurisdiction.
So, we flicked the issue to the eight State and Territory Governments who had legislative responsibility for in-service safety of only the vehicles they had registered in their particular state/ territory, and thus they wore the legal responsibility one way or the other. So, we now have a situation where the rules differ from one State/ Territory to another, depending on what State/ Territory number-plates are fitted to the car.
Now I am not at all up to date, but the attitude does vary from one State to the next – generally our ‘wild-west’ states – WA, SA and NT couldn’t care less, but the eastern states are a lot more conservative, even so, and I stand to be corrected, I think only Victoria has so far proceeded to legislate a State requirement of maximum 10 year life of tyres, albeit that was a compromise, as certain segments lobbied for a 5 year life maximum; but with one State capitulating, the pressure is now increased on other State’s to follow suit, albeit I can’t imagine the wild-west will ever care, and they have a bloody-minded attitude at the best of times re anything coming from Canberra (The Federal Government or the local Australian Capital Territory) or the major Eastern States of Victoria and NSW, or indeed our ‘deep south’, that just happens to be Queensland in the North.
IN a Federal Government sense, apart from regulating NEW Cars (with thus their new tyres), Federal Legislation also applies to used second-hand cars being imported to Australia for FIRST use on Australian Roads, and in that case the formal schemes that certify second-hand cars as being acceptable for use on Australian Roads simply requires NEW tyres to be fitted, regardless of condition/age of its imported second-hand-tyres. It’s easy in a legal sense, fit new tyres regardless!.
Other second-hand imports, such as eligible Historic (pre 1989) cars get a Federal Rules exemption, so again its left up to the State’s/ Territories what they require for first registration, and that’s usually a visual inspection of wear/damage, except in Victoria where 10 year limit is applied.
So I don’t really know what to advise Classic Car owners who have barely worn, but aged tyres – the science just does not support arbitrary decisions based on age alone re tyre safety. Certainly rubber hardens, and that will affect grip levels wet and dry, but that varies anyway from one brand to another, one size to another.
I think if I was a Concours buff I would only be concerned if visible tyre condition deteriorated (cracks in sidewalls) such that I lost points for condition – how many judges know how to age a tyre?
If I just did Sunday drives around town at low speed, I probably wouldn’t care either apart from a regular visual inspection, and making sure I had a serviceable spare and wheel-changing equipment.
But if I was into long-distance remote/outback driving, or high speed back road driving, I would be much more circumspect re the implications of a tyre-failure at speed or remote areas.
If racing – there are other considerations that matter more.
Or if I lived in a State that did introduce arbitrary tyre-age laws, I guess I would have to comply, or risk a fine – if indeed there was much risk of detection. Is a copper really going to book an XK120 driver with otherwise road-worthy looking tyres fitted, by working out the codes that might say the tyre is only 9 years old or a shock-horror an unsafe 11 years old! Depends if he is on a quota or a mission or a Head Office crack-down I suspect, or if he saw you doing burn-outs so wanted an easy excuse.
And YES, a tyre Speed-Rating, is its ‘Sustained Maximum Speed’, and not the maximum achievable speed, but then in a legal sense it all depends what the relevant State or Federal Legislation says. Most legislation nominates the arbitrary tyre-rating must exceed the maximum-achievable-speed, as how can you possibly regulate/enforce ‘Sustained Maximum Speed’ The legal instrument is usually the TYRE PLACARD (and what paperwork is behind it) required to be affixed to any car – but these did not exist when XKs were built, being introduced in USA and in Australia in the 1960s, ahead of UK and Europe who followed in the 1970s (?), and the Japanese/Koreans only when exporting cars to the previously mentioned regulated countries – in Japan, they legislate to get rid of the entire car at 5 years of life (or pay a heavy ongoing price for cars worth keeping any longer), and not just the tyres – keeps their factories building at capacity!.
And PS. My tyre of choice on both 15” E-type and 16” XK is Michelin, albeit the Michelin XVS my preference, is only made in 15” size, but next set of 16” tyres I buy, to replace my current ¼ worn only 20 year old Firestone radials (Car under restoration) will probably be a set of Michelin X for ‘go’ (on 60-spoke CWW), and a set of Dunlop RS5 (cross-ply as original) for ‘show’ (on original 54-spoke body-coloured WW).
And I most definitely will not fit wider and/or low profile tyres to either car, as their wheels and suspension were designed as a package with nominated standard tyres – even fitting radial tyres give side-ways grip levels in excess of original cross-ply tyres, such that 54 spoke 5”x16” wire wheels were strengthened in 1958 to have 60 spokes, and E-types went to 72 spokes in 1961 – for a reason!