I have a 2008 Subaru Outback 2.5i and I need new tires. My current tires are P225/55R17 95H. Searching around, I found some Yokohama Avid Ascend tires, which look to be great touring tires, especially in the winter - but they are only T-rated (118mph) as opposed to my current H-rated (130mph). The dealer is very wary of installing because he would be going below the current rating. I am not a crazy driver - I might get the car up to 75 on the freeway. Is there really any concern here about moving to a T-rated tire?
Are the new tyres of the same load rating (the 95 before the H) as the recommended ones?
The primary issue with using a lower than recommended speed rating is legality, particularly relating to insurance. Obviously this will vary according to which country/state/jurisdiction you are in, for example in the UK you must declare this to your insurance company (as they consider it a modification from standard specification), and failing to do so would invalidate any insurance policy and leave you open to prosecution for driving without insurance.
Do not go under the LOAD RATING. Use the manufacturer's recommended load rating as a minimum, and do not go under that minimum. In some cases, you will "feel" the car ride differently, and it can even begin to handle dangerously (wallowing in turns, back-end kick-out, etc). This can increase risk of spinout a high speeds, as well as increasing rollover risk.
Speed rating does not matter so much - but load rating is crucial. Often - they are paired, so that picking a lower speed rating puts you at a lower load rating also.