in my 1991 XJ I installed new meter overlays. it covers the old meter faces which were in KM. I now have MPH and they look great. the problem is, that the Speedo is way the heck off in reading now, I know that when it is up at the 1:00 position that was about 120KMPH, but with the new meter faces it is at 55mph, which is not accurate. what do I have to change to make it read correctly for MPH instead of KMPH ? is it the gear on the end of the speed sensor ?