What speedometer gear to use for a 3:54

O

oldford

Senior Member
A year ago I put 3:54 gears in my 34 and was told that the in order for the speedometer to work accurately, I would need to put an 18-tooth gear in the speedometer gear turtle. It seem to work fine. Now I’m putting 3:54 gears in my 35 and would like to modify the speedometer gear turtle, but I’m confused. My 35 manual states that the drive gear turtle should also be 18 teeth. How can that be the same as a 34 since the 34 car has bigger tires then the 35. I would think the diameter of the tire would make a difference in the speedometer reading.
 
The RIM might be an inch larger in diameter on the '34 than the '35, BUT the tire itself on the '35 has a larger cross section than would be on the '34. I'm not certain about the numbers, but I'm pretty sure that the rolling diameter of the tires would be pretty much the same for both cars.
 
18 tooth is the smallest speedo gear that I know of. If you are running larger tires than the factory size with a 3.78 gear, you would run an 18 instead of a 19 tooth gear. If your tires are larger than stock with your 3.54 gear, an 18 is all you can run.
 
I am in the process of trying to get my speedometer to match the GPS. 3:78 with a Columbia 28% OD. I have a 18,19,20,21 and 23 turtle gear and even counted the gears in the turtle to make sure no one changed them. Going down a number should raise the speedometer readings by about 8 MPH I think. will let you know the results. 19 should work so I will see. There is also a speedometer adaptor which changes the speed the speedometer reads this makes it even more interesting. It's fun getting my big hands up around the transmission to change the turtle from below but I figured a way to do it. G.M.
 
Thanks for any help here. Don
Tubadon40 Posts: 4Joined: Sat Dec 27, 2008 11:06 pmTop
Re: What Speedo gear to use?

by jdl on Sun Aug 09, 2009 6:53 pm
This is from the 49-50-51 Ford shop manual..

speedogears.jpg
 
Going by the table, it lools like a tire's diameter does not make a difference as long as the tire is only 1" bigger, thanks
 
Important to note that as the number of teeth on the turtle gear is reduced, the diameter of the turtle gear is also reduced to be compatible with the gear on the driveshaft, which are (generally) all the same except for some weird gears for trucks??? Accordingly, the correct turtle housing must be used with the 18-tooth gear or the gear will not reach/connect/mesh with the gear on
the driveshaft. HOWEVER an 18T gear WILL work in a 19 (or 9-34) housing,
especially if a gasket is not used, just sealer.

I have a (very) vague recollection that "someone" was offering a 17T gear, which would have to go into an 18 (11-39) turtle. OR someone posted on the Barn a few years back that they had set up to mill the turtle housing to be
correct height for the smaller gear. I need a 17T gear myself, as I am running
7.00x16 tires on my 34 and with the 18T gear/turtle the speedo reads about
5mph slow at 60mph, which could be problem regarding the "laws"...
 
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