That article is ludicrous - getting worked up about 1 or 2 MPH. The difference between the circumference of a new 15" tyre and a tyre that's worn to the legal limit is 4%. That will translate to a speedo error. Note also that the error is likely to be worse than that anyway - the tyre is compressed at the contact patch, reducing the effective circumference and thus increasing delta C / C. And all this is supposing that your tyre is correctly sized and inflated to begin with.