if my math is right, a 5 tooth change made a 12 MPH reading change so a 39 (your only choice really other than a gear reduction adapter) so 12 divided by 5 is 2.4 (MPH) so a 39 toother (if there is one and you can locate one) would theoretically slow the speedo another 2.4 MPH & put you very near dead on

live every 24 hour block of time like it's your last day on earth