Here is a great link for anyone that has changed their axle ratio or have gone to very large tires. With this link you can enter in the info to calculate which is the correct speedo gear for your transmission to keep the speedometer accurate. I have used a number of times so far.
If you do need to change a speedo gear the dealers have most colour sizes available. An exception is the Ford 23 tooth white gear for 3.75 to 4.10 axle ratio which is available on line. The only issue I had with this gear was that the shipping was more than the gear. Go figear!
