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I dont know the answer to your question. What I do know is, I talked to the manager over at "Crow Harnesses" and he said the 2" harnesses are used for sandrails and the 3" ones are used for Roadracing. He told me he doesn't remember the last time he made a 2" set for a car application.
SCCA and NASA allow 2" lap if they are FIA rated and part of a 6 point
I talked to a couple of vendors and was told that no one in the USA makes a FIA rated set
so I guess I'm going to go with the 3" lap even tho I really like the 2" better (from autocross use)
Three inch belts are probably on their way out. For lap belts they have found the 3 inch belts do not fit the groove in the pelvic girdle properly since it is only about 2 inches wide and cause crash forces to be incorrectly distributed across the pelvic girdle. As for shoulder harnesses the 2 inch belts seem to work better with the HANS devices.