The refusal to install any tire with a speed rating less than what was supplied OEM at many tire dealers is solely one driven by liability reasons. For that, you can thank the lawyers and insurance companies.
But aside from the actual top rated speed of a tire, there is some basis on where this comes from. Generally speaking, S rated tires will more or less have the same "feel" in terms of sidewall rigidity as the next S rated tire. Same goes for H. Or any other speed rating that you can pick. Generally, the lower speed rated the tire, the squirmier and squishier it will feel. Add to that that speed ratings of H and higher require a fundamentally different construction technique. What can happen is that in an emergency situation, a driver who is used to the way the car reacted on the H rated OEM tires, but is now driving on replacement S rated tires may get an entirely unexpected response from the car, which could lead to an accident.
So does this mean never replace with a lower speed rating? Well, maybe or maybe not. From the insurance and lawyer perspective, the answer is clearly no. From a realistic and practical perspective, the answer is not so clear.
I have a perfect example of a car from my past: a 2002 Mazda Protege. It was sold in 3 trim levels, with even more sub-variations within the trim levels. Everything mechanical was identical, save for the base engine. I am not talking about any of the performance models - only the mainstream ones. Neither engine could realistically get that car to go much faster than about 105 mph on a good day, with a tail wind downhill. Tire options spanned three sizes: 14", 15" and 16". The 14" tires were some S rated Firestones. The 15" were V rated Bridgestones, and 16" also V rated (Dunlops I believe).
Let's say Grandma has a top of the line that just happens to come with the 16". (And let's leave out the fact right now that it is likely impossible to find a low speed rated tire in low profile for theory's sake). Grandma drives like, well, a grandma. And she now needs new tires. Given the way she drives, and the fact that the car actually left the factory in some iterations with S rated tires, there is no reason why she couldn't drop down, and she would not see a compromise for her safety. But let's say 17 year old Billy also has the same car. He drives like your stereotype of a teenaged boy, and he now needs new tires. Drop to an S in this case? I would say no, because he has pushed this car to it's limits on V rated tires, and he will leave the tire shop lot on S trying to do the same thing. There is no way the S rated will have anywhere near the performance characteristics of the V rated tire.
The rest of us likely fall somewhere in between. Add another layer to that now: the lowest speed rating you are likely to find in low profile tires are H. If the majority of us owned that car, the H would likely suffice.
Fast foward to 2011, and I bet that the majority of cars that back in 2002 would have had a trim level leave the factory with S or T tires, would now have equivalent cars leaving with no less than H for the North American market. Perhaps the suspensions were "tuned" for these new tires. But really, how likely does your Hyundai Elantra, Chevy Malibu or Toyota Camry have the same degree of suspension tuning as a Corvette or any other high performance vehicle? I suspect it is more a marketing gimmick than anything. Realistically, if the type of use of the car is such (most likely the grandmas), then going to as low as S probably works, though now I wouldn't chance it for the majority of us, save going down to an H rated from something riduculously higher rated, which I am certain the majority of cars will never benefit from the full potential. (BTW, in some markets outside North America, the exact same cars would be sold with S rated tires, and depending on the type of car, it's unlikely the manufacturer spent money to engineer two different suspensions).
Then there's winter, a whole different ball game. I am hearing may shops are now applying the same liability rules for winter tires, but the theory for dropping speed rating here is different, and multi-part: (supposedly) lower speeds driven in winter, meaning also less severe of an emergency manoever would be needed. Also a trade off of benefits: the increased ice and snow traction of a lower speed rated tire far exceed the benefit of the handling characteristics of a higher speed rated winter tire for most applications.
Of course common sense could now all be moot anyway, if the lawyers have all lead us into a world where tire shops fear lawsuits.
