You mis-understood - WHATEVER diff you put on, the speedo will still read the same. I.E. 7k will always read 170-180mph (depending on how accurate the speedo is), even if you have the short diff on, meaning that you are really only doing a real 158mph at 7k and not what the speedo says. This is because the speedo drive is taken off the gearbox revolutions and and is calibrated for a 3.64 ratio diff. So although you may alter the diff ratios or tyre size which will change the ACTUAL speed, the display speed will always remain the same. Hope this explains what I mean so you understand.