Tyre speed ratings (slightly OT)

Not really classic territory but perhaps the knowledgable peeps on here might be able to shed some light on this... This is about theoretical maximum speeds and 'speed' ratings for tyres ( R,
HR, ZR, etc for radials & FKW for crossplies). A few years back there was a seemingly unresolved debate about fitting agricultural dumper tyres to Range Rovers & Landies. The tyres were (typically) marked with "Max Speed 30Mph" The great debate was about wether this meant they could only be fitted to a vehicle not capable of exceding 30 Mph, OR, a vehicle capable of more than 30 Mph should not exceed 30Mph if fitted with such tyres. Does the 'speed' rating of the tyres set the maximum road speed? Or does the vehicle's theoretical maximum speed determine the tyre speed rating? In this case we are talking about 750X16 crossply 'dumper' tyres fitted to a Discovery. Obviously we are talking about very limited road use but would it be legal to use them on the public highway if the tyre's rated speed was not exceeded? Hope you can make sense of this...
Simon H
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Simon H wrote:
    [...low speed tyres...]

Not sure in this instance but it is legal to fit lower, but greater than NSL rated tyres to a vehicle capable of higher speeds in the UK, but not in Germany[1].
Whether this remains true when the tyres are not rated for at least NSL I'm not sure.

Ditto. :)
A
[1] If winter tyres of lower speed rating are fitted a warning plaque must be placed within the drivers field of vision.
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You would probably also need to tell the insurance company. I recall a local case (many years ago) where the owner of a car which was supplied new with what were then HR-rated tyres replaced them with SR-rated tyres. He had an accident at around 30mph, writing off the car (3-litre Capri IIRC). The insurance company inspected the car, saw the SR tyres, and refused to pay up.
--
Regards, Andrew.
Andrew Marshall, G8BUR, M0MAA.
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writes

That was the kind of story we were hearing the last time I was 'concerned' about this issue. Somebody had always 'heard' about a similar story but we could never find any documentary evidence to back it up. At the time (about 10 years ago IIRC) even a 'tame' traffic cop didn't have a clue...
Simon H
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(re: use of lower speed tyres)

I don't doubt it. I simply reported what I was told.

The information came by word of mouth via someone I know who has contacts in the motor trade. I doubt that documentary backup would be available to the casual enquirer, if at all.

Doesn't surprise me...
--
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Andrew Marshall, G8BUR, M0MAA.
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--snip--

(about
Well, from my own experience a couple of years ago.... Some nice person decided he wanted my car radio more than I did, so he helped himself via the (closed) window. An insurance assessor came out to inspect the damage, and I was puzzled that he started by inspecting the tyres. I asked why, and he said that if the tyres were worn or of the wrong type the car would be classed as unroadworthy and the insurance invalid.
FWIW the claim was accepted, but after depreciation, first 250 etc was taken into account I was offered a whole five pounds as full and final settlement. Oh, and I would lose my no-claims bonus.
Three guesses why I did not renew with Norwich Union?
Geoff MacK
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Good question. That the vehicle may well be considered inappropriate or unsuitable for road use. We prosecuted a few people in Lincolnshire for using unsuitable tyres but I was only a special, so I didn't get involved with the nitty gritty, only checking the tyres...

The idea behind the speed ratings isn't just about the maximum speed of the vehicle, but the load that the tyre is subjected to - acceleration, braking, cornering. Many cars use a higher speed rating than their performance would dictate. The higher the speed rating the "better" the tyre, and the tyre's speed rating must always exceed the vehicle's maximum speed.
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wrote:

Or the worse. It's no longer such a problem, but 20 years ago, choosing a high-rated tyre meant one that wouldn't shed rubber under the strain, but that it also had lousy cornering. Just getting it to work at all meant compromising with too hard a rubber compound.
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Yes. Many years ago I bought a secondhand Rover P6 3500 which had two new Cinturato SR tyres on the back. And within a few miles, chunks of tread came off. Pirelli had at the time a depot close by in Mitcham, so I took it there. They said the tyres were unsuitable for the performance of the vehicle as Rover specified HR Cinturato which had a totally different design. For goodwill, they sold me 5 brand new ones at factory price. Quite a bargain in those days. ;-) One interesting thing was that not one of the wheels needed balancing with the new tyres.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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<snip>

Quite a

of the

With high quality brand tyres, correctly fitted, that is not at all uncommon - trouble is, many monkeys masquerading as tyre fitters couldn't fit tyres correctly if their lives depended on it.
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Aye - ours may do, though. :-/
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Think you'll find the tyres have to be of the same - or higher - speed rating as those fitted by the maker.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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