I recently changed my speedometer gear due to a rear end change. I found the following information out while deciding what speedometer gear I needed. Almost all sites point to an axle ratio to tire size chart that lists the corresponding teeth count such as below; I queried tiresize.com and looked up my 215/70/R14 tires which are 25.9". So, in my case the 3:23 axle ratio with a 26" tire requires a 33 tooth gear according to the above chart. BUT, not convinced I looked in my 1969 Service Manual and on Page 21-96 found a speedometer pinion gear chart as shown below; I then queried Vintage Tire Size Conversion Chart. This chart provides different tire sizes used during different eras, ie. pre 1964, 65-72, etc. I found that my 215/70/R14 tires were equal to a 7.75x14 or an F70x14. If you look at the service manual chart above you will find that this tire with a 3:23 requires a 32 tooth speedo gear. Not a 33 tooth as the first chart suggested. So I chose the 32 tooth gear and have found that my speedometer is exact with my GPS. Many of you may have to convert your 16, 17 or 18 inch tires to a usable 70 series 14 inch or 15 inch tire in order to use the Vintage Tire Size Conversion Chart to correspond with the Service Manual Chart. This can be done by going to the tiresize.com website and using their comparison chart, OR their tire size chart. Enter a tire size and you will see tabs to choose comparable 16", 17", 18" etc rims. You may have to do the comparison more than once, meaning enter your 18" tire find the 16" comparable, enter the 16" to find the 14" or 15" comparable. Then use that tire size to find the Vintage Tire Size Conversion and find your speedo gear in the Service Manual Chart. It's a little work but may answer why you've been thinking your speedo is out a bit. I hope this helps someone in the future.