The THA code comes up as a '71 C10, 3.07:1 ratio. I think CASO is right, try checking it counting the revolutions. Something doesn't add up if you're winding it out, especially if it does have the ratio the code says it should have.
I do have a question that might be relevant to the situation; What size tires are you running? A small diameter tire will be similar to putting a lower gear in the diff.
