Direct from the horse's mouth today (a contact who's a collision investigator) -
You can be prosecuted (and under some circumstances imprisoned) for fitting tyres with a speed rating below the design speed of the vehicle.
To clarify that, the construction and use regulations are ambiguous, stating that the tyres must be suitable for the purpose to which the vehicle is being put.
The Motor Vehicles (approval) Regulations quoted by IanA above (and which I'd never heard of, so well done for finding them!) however, state that the tyres must be speed rated not below the maximum design speed of the vehicle.
Where the Courts are concerned, if two pieces of legislation seem to contradict each other, it's the one that hangs you that stands.
According to my contact, if you fit lower speed rating tyres to your car than it should have, the chances are nobody will ever notice (I don't believe it's even checked on the MOT).
But, if you are involved in a serious injury or fatal road traffic collision, your vehicle will be checked over in fine detail by a trained collision investigator, which includes checking the speed rating of the tyres fitted to the car against its design speed. If they are found to be incorrect, it is highly likely that you will be prosecuted for dangerous driving or causing death by dangerous driving, and such cases have been successfully prosecuted in the UK.
So, I would strongly recommend that you fit the correct (or higher) speed rated tyres to your car (I certainly will be) as the 'wisdom' on the net that you don't have to is wrong.
Where the Z3 donors are concerned, this means H rated tyres on 1.8, 1.9 and 2.0 models, V rated on the 2.2, 2.8 and 3.0, and W, Y or Z rated tyres on the 3.2M models.
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