Speedometer Gear Question

[Robert Crandall]

I am working on a Saginaw 3-speed with overdrive that I will install into my truck when I get it ready. I do not know the year of mine, but I believe that these were made between 1966 and 1970. I do not know what differential ratio it was set up for, but it has a 24 tooth driven gear, natural colored. The charts that I have found do not show 24 teeth as a choice. I have been scouring forums, but I cannot find anything that I can understand or follow.

Can anyone direct me to information that will show me what driven gear I need to use this transmission with a 3.73 differential? Thanks!

[pjmoreland]

Are you able to determine the number of teeth on the drive gear?

[Robert Crandall]

Looking into the hole shows 6 on the spiral gear.

Edit: Looking at photos of replacement spiral gears showed me counting across the width shows 6 for all tooth counts. I marked one of my teeth then rotated the shaft. That shows my gear has 8 teeth. It looks like it is metal. The replacement ones look like they are plastic.

Thanks!

[pjmoreland]

What's your tire size?

[Robert Crandall]

P235 70R 15 Measurement center of wheel to ground is 13 inches for 26 inch diameter as loaded.

[Robert Crandall]

I found a calculator of the name Moveras. Insert image here requires a url, which I don't have for my photo. It shows a 21 tooth gear when I follow the instructions. When I play with the width until I get close to my measured 26 inch diameter, the calculator shows 23. My existing gear has 24 teeth. I have not yet found that choice yet to buy. Maybe I will buy a 23 tooth gear and try it.

[pjmoreland]

If I use the calculator here, your tire diameter is 28", and it recommends a 21.5-tooth driven gear. A 21-tooth gear will make your speedometer read a little high, and a 22-tooth gear will make it read a little low.

[Robert Crandall]

Thank you very much for your help. The calculator that you showed gives the same answers as the Moveras one that I found. My tire does measure 28 inches diameter unloaded. However, it measures 13 inches from the center of the wheel to the floor. I'm not sure how to account for that in a calculation.

Do you have a recommendation for me to check which speedometer gears are compatible with my 8 tooth drive gear?

I converted my 1950 GMC to a Saginaw straight 3-speed with a 3.08 differential around 1978. That transmission was originally paired with a 4.10 differential. I left the original drive gear and speedometer gear in place. I soon learned that my speedometer was around 15 mph off. I have been happy with that for more than 40 years.

This new transmission is apart on my bench. All is new or good inside, but the speedometer gear does not look as good. I look into replacing it while I have the transmission working, so while I am at it, I look to maybe change it to a gear that will more closely match the 3.73 that I also plan to install.

[pjmoreland]

I don't think you need to take into account how much the tire is squishing where it contacts the ground. In the end, the entire perimeter of the tire will contact the ground during one revolution, and the fact that it is squishing at one point won't change circumference of the tire. You could run a test where you put a vertical mark low down on the sidewall pointing straight at the ground. Make a corresponding mark on the ground. Then push your truck forward far enough to rotate the wheel one revolution, and then make a second mark on the ground lined up with the mark on your tire. Measure the distance between the two marks. Divide the result by 3.14 (pi) and you will have the diameter of your wheel.

I am not familiar with the types of speedometer gears that go in your transmission. Hopefully someone who is will chime in.

[Accelo]

A picture of the speedometer gear and the corresponding driven gear may help.
More than one design was use and some gears were steel some plastic.
Even the same tooth count drive gear had two different diameters and was incorporated into the mix.

[Robert Crandall]

Thank you for your information. I searched for a 24 tooth gear, found one, and bought it. It should arrive next week. It will be the week after that before I will be able to look at what I have. Again, thank you for your help. I will update in a couple of weeks.

Edit: my shaft has a steel gear. I am not interested in changing that to plastic.

[72SB]

OP. They make both large diameter and small diameter gears which need to be paired correctly. This link can better explain what drive gears work with what driven gears. "Generally," 1 tooth on driven gears accounts for about 5mph difference. For example, if your speedo is off 10mph at 50 mph, get a driven gear 2 teeth +/- depending on if it reads high or low.

This assumes your trans has the "bullet" type speedo gear holder (driven)

http://www.stl-vettes.com/65Vette/GearCalc.htm

[Robert Crandall]

Thank you for your information. I used the link to more confuse myself. I cut cardboard to measure the length of the spiral drive gear. It is steel. The link is for Corvette and works for options measuring 0.610 and 0.531 wide. Mine measures closer to 0.8. My transmission is a truck version, so that may be why it is different.

Putting my 3.73 ratio into the chart with 8 tooth large drive ring gives a 23 tooth large driven gear needed. However, the bottom of the link shows in red letters that using a large driven gear with a large drive gear will be a problem.

The 24 tooth driven gear that came in the transmission does not have tooth damage, so I bought another one of the same. I am confident that I will not tear anything up with that choice. When I get it all finished (months away) I will see how I like what I have.

Again, thank you for your help.

[geezer#99]

I bypassed all the searching for driven and drive gears 40 years ago.
I found it much easier to just find a ratio adaptor that made my speedo accurate.
One like this.


https://transmissioncenter.net/shop/...ratio-adapter/

[Robert Crandall]

Thanks for that advice. I will hold off on doing anything more until after I have it together and am driving it again.