Hi Ed, the thread where the tyre speed rating was being discussed is for a car where the modern donor can run up to a (limited?) 155mph top speed, and people are already talking about turbos and superchargers.
Obviously, the performance potential of those cars plus the easy access to information about the donor vehicle makes it easy for a collision investigator to check whether the tyres fitted are correct for the car.
Where 'classic' based kit cars are concerned there are two factors at play - one is that the data isn't available off the shelf to check, and the second is that pretty much nothing that got used as a donor back in the day could exceed 120mph anyway.
I'd say that a Rover V8 Marlin (with about 150bhp?) would, depending on the gearing, be all done aerodynamically by about 110mph, and more power wouldn't necessarily add much to the top speed, just the acceleration. As such, you could put hand on heart in Court and state that the H rated tyres you fitted were definitely suitable for that vehicle.
The current BMW based Marlin Sportster, however, would quite possibly need V, W, Y or Z rated tyres.
As for availability, generally speaking, kits using older donors (Sammio/Miglia) use classic wheels and classic tyres are available to suit, while more modern donors (Z3/MX5) tend to use modern wheels and modern tyres are also available to suit.
After two years driving my Spyder on modern roads my advice would just be 'don't skimp on safety'.
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