I'd check with your insurance about that, I had a tuned up car many years ago and I HAD to have V rated tyres or better fitted or my insurance was invalid. There is no public road in this country that I can legally do 149mph on but that was what rating tyres I had to use. It's not just about top speed but the other loads that the tyre can be subjected to, otherwise no car in this country would need anything better than a L rated tyre.You can also derate the speed rating to save money because the speed limit in this country is 70 so apparently even if the manufacturer specifies a 140mph tyre, there's nothing to stop you fitting a lower speed rating tyre.