Marge +1
I was under the impression its OK to use a tyre rated one speed rating below normal for special conditions tyres, such as studded tyres or offroad tyres. Think in some cases the vehicle should be fitted with a sticker to remind the driver?
Tron, I don't think grip has anything to do with it, its the ability of the tyre to not overheat at its rated speed and therefore fall to bits! Tyres with more aggressive tread patterns are more prone to heat build up and thats why for certain applications such as a 3.5t van, you can't get a descent off road tyre that has a sufficient load and speed rating. I had to go for a meaty looking 'standard' mud and snow tyre for my van. Drop a couple of load ratings and I could have got something that looked like a porcupine 🙂