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ST tires are new at having speed letters and having the testing ability to sustain the speed letter displayed on their sidewalls. Almost all of them now have ratings at 75 MPH ("L") or higher.
To be allowed to sport the DOT on the tire sidewall, somewhere along the line each designated size was tested.
It's my opinion that the DOT would not allow speed letters on tire sidewalls without passing their prescribed testing. Currently the prescribed testing is explained below.
DETERMINING TIRE SPEED RATING:
Speed ratings are the product of laboratory testing – with simulated speeds and loads. To receive any kind of rating, a tire must demonstrate that it’s capable of sustaining a particular speed. Industry standards govern the process of reaching and maintaining a given speed during a test.
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