Although it is a handy reference and should get you in the ballpark, you really don't need that chart to determine the speedo gear you need. Your GPS, the current speedometer readings, and some simple math and you can nail it down closer than using that chart.
You already have 1 reference point, GPS 40, car 30. Take a couple more reference readings, like at GPS 20 and 60. Then compute the ratio between the GPS readings and the car readings. The corresponding ratio is then applied to the number of teeth on your speedo gear. Of course you need to pull your current gear to see how many teeth are on the one in the car now. (maybe a 45 or 46 tooth?)
Looks like your speedo is about 25% slow, confirm that with two more GPS readings, then get a gear with 25% less teeth on it than what you currently have in the car.
I did this on my car and nailed it dead on the first time. Chart was indicating a 36 tooth, but my math told me 37 tooth was what I needed. Of course you need to be as accurate as possible with your GPS and car speedo readings to derive the correct ratio for the number of teeth on the gear.
I also have 3:55/1 rear, tires maybe about 26.5". 37 tooth was perfect for me, but they are harder to find as they are no longer produced. The 36 tooth is still in production and available from Chrysler, at least it was 3 years ago. Also they are available on Ebay all the time.
Good luck
