There have been a few inquiries lately on these gears. I just went through changing the gears on my 39 convertible to get the speed right. First off I had a self inflected wound that threw the speed off. Before leaving Florida I took off the vacume operated speedometer changer and installed an electric one. In doing so I got a little fuzzy and wired it wrong, it changed but it wasn't right. I got into it last week after changing the turtle gear 3 or 4 times. I can now take them off in less then 5 minutes from under the car. Practice make perfect. I have a 18, 19, 20, 21 and a 23 turtle. The housings are all differant on each number. As the numbers get higher and the gear gets larger in diameter the distance from the housing base mount to the center of the shaft the gear runs on changes. The 23 gear housing to the center of the shaft is .100 or almost 1/8" further from the base than the 18. Trying to put the wrong gear in a housing means the teeth don't mesh properly. One poster stated he couldn't use a gasket when he changed a gear. The gasket is .008 thick and when the gasket makes a differance there isn't much tooth engagement. Even if you grind, file or whatever to make it work some time in the future someone is going to get confused and spend a lot of time not knowing what they really have. For this reason I marked a tooth on all the turtles and counted the teeth on each one with a cable square end in each so I could see they turned 1 complete turn when the 19th tooth came around on a 19 turtle. Mine worked out as the green book shows, a 19 turtle for a 3:78 rear. Below 45 the speedometer reads about 2-1/2 miles slow and from 45 to 65 it is within a mile of the GPS speed. This is both in and out of Columbia so my speedo changer is the right 28 % changer. I suspect it will get closer at 75 but didn't have a good road for that. We will pin the 1 mile off on tire wear??? Never thought it would get that close. G.M.
