Earlier quoted context omitted.
As you might note in that link, the states already had their own speed limit laws in place, and that the national limit was later repealed. Thus, the speed limit laws are not in place to reduce gas usage.
Even a single exception (which I furnished) is sufficient to invalidate the claim I quoted.
To fight pedantry with pedantry, I refer you again to my comment, which points out that exception no longer exists, and the original claim, which is in present tense.