My 74 Duster with a 904 in it - the speedo runs 4 mph slow. - actual 50mph reads 46 on the speedo. I obviously need a speedo gear change. Does the 904 and 727 take the same speedo gears? What is the approx. speed reading change for a one tooth change? Assume I have to go to a speedo gear with at least one tooth less than existing? Thanks, John