Changing speed rating of replacement tires
2012 Grand Caravan came with H rated Kumhos. Thinking of replacing with Cooper CS4s. Tire dealers said T-rated is OK. Is it?
The short answer to your question of whether a speed rating of "T" is OK is: Maybe.
The reason I say the speed rating might be OK is that it depends on you and your driving habits, not on what a dealer tells you nor on what tire the vehicle had installed when you bought it.
The T rating is saying that the tire is designed to withstand speeds up to 118 mph or 190 km/h while an H rating is designed to be driven up to 130 mph or 210 km/h.
Now, no one knows your driving habits better than you. If you believe that you're unlikely to ever drive for a sustained speed of over 118 mph, then the dealer is absolutely correct.
Please note that the tire manufacturers do not recommend that people drive at these maximum speeds, nor that the tires are capable of supporting these speeds if they have been previously damaged and possibly repaired. These are simply the speeds at which they have been tested in a laboratory when new and have proven to be capable of sustaining at least 10 minutes at that speed without suffering any damage.
The tires which were initially installed in the vehicle may have been chosen to meet or exceed the original capacity of the vehicle by the manufacturer of the vehicle, but it is also possible that the dealer that sold the vehicle may have chosen this. You do not have to match the original equipment if your needs are different.