In my 40years of driving I’ve had plenty of “incidents” involving the police and insurance companies, never once has there been any problem over tyre speed ratings.
I’ve never met anyone who has had a first hand problem but have met loads who have “heard” a tale of someone not getting paid by an insurance company or getting prosecuted.
Personally I believe it is a myth that manufacturers of Z rated tyres are keen to perpetuate.

Its not something I've found to be really clear cut. I guess the only way is to speak with the insurers on own circumstance.

Considering that you're not really likely to be driving beyond the rated speed of the tyre ... to me its more important to ensure you're fitting tyres to match the needed load rating. You're far more likely to exceed the load rating at low speeds than to exceed the speed rating at high speeds.

Added to the that, the owners manuals don't help either.

For example, on the supercharged 4.2 manual I have, it describes that the normal size road tyres should be H/V - rated, but that if you fit accessory fit off road tyres which are q or t etc, then you should limit your driving speed to not exceed those speed ratings.

So to me that implies that the fitting of lower speed rating tyres, on the condition of adhering to the limit of them ... is manufacturer approved ... so shouldn't be an issue with insurance ...

I did go through this a few years back when getting my first set of winter tyres and mentioning it to the insurance, and I explained that they had a lower speed rating, but the load rating was fine. My insurance didn't seem bothered. V8 or else ...

Since 2004 I've run first Goodyear MT/R and then Goodyear Duratrac as winter tyres. Speed ratings below the rated speed for the car, in the current case 99mph only. As others have pointed out this sits uneasily with the Construction and Use Regs in the UK. But these are LR homologated tyres, and the 405 SC has different ratings depending on whether one has 22" or smaller wheels. The Disco SVX which uses the 5.0SC motor is rated 99mph.

When I was looking at getting my 405 5.0SC I emailed Goodyear and asked about the suitability of the 255/55R20 Duratrac. They said they were OK provided one stuck to the tyre's speed limit, which I am happy to do. I drive more gently when I am using them too. I've not set the car's speed limiter, but one could do that. I do tell my insurers that I use off road tyres in winter, there is no extra charge and they don't seem to regard it as a problem - they might well be happier with me driving gently on Duratracs than exceeding their speed limit on the Pirellis... Only Range Rovers since 1988

When we had MB ML the manual suggested/instructed setting the speed warning at the Winter tyre limit when fitted, so I guess that covers the requirements? David

Please let me know if my comments upset you, I may wish to upset you again.