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Normally I don't create a thread to announce new web pages. But, in this case I need your thoughts and suggestions.
I've put up three new web pages to cover speedo gears, both drive and driven. The main page is here (Speedometer Gears - Gary's Garagemahal). But, I didn't realize that the Driven Gear tables also list the Drive Gear until I had the cutting and pasting done to create the Drive Gear page. And, since there are 27 separate screen grabs on the Drive Gear page I'm loathe to delete it. However, if y'all think it is confusing please tell me as I could meld the two pages, Drive and Driven, into one longer page and leave out the year-by-year Drive Gear tables. On the other hand, it does provide a different, and in some ways easier, way to look up the Drive Gear #'s.
Anyway, the main page says the following, including links to the daughter pages:
To me the speedometer gear situation is somewhat complex, so I'm hoping to lay it out here in a way that simplifies it somewhat.
There are two gears involved in the gearing to the speedometer:
Drive gear: This is the gear that is on the output shaft of the transmission or transfer case, and it is named "drive gear" since it drives the gear on the speedometer cable.
Driven gear: This is the gear on the speedometer cable.
So, what gears need to be used to get the speedo to read correctly? "It depends" is the best answer as there are two variables involved - the rear axle ratio and the tire size, or more correctly the tire's rolling diameter. Given the number of tables involved I've put up two separate pages for the two gears:
Drive Gears: This page lists the drive gears used for each year, transmission or transfer case, and axle ratio. But, the Driven Gear page also lists the drive gear in the tables, so you can look the drive gear up there as well.
Driven Gears: This page lists the driven gears used for each axle ratio, specific transmission, and tire size. But it also includes the drive gear part numbers, so you can get both part numbers in one table if you prefer.
Upper picture: One would think that for a truck parts catalog that had the final printing 3/1994, the colors of these various DRIVEN gears would be listed.
But oh no, one has to look elsewhere...
Lower picture: DRIVEN gear ID chart from 1973/79 Ford car parts catalog (final printing 11/1988).
Bill - Thanks. However, the trucks take the 2a gear and your chart doesn't have them. Might you have another that does? That would be very helpful, indeed.
I only see two matches: C0DD - A is Wine colored, and C0DD - B is Gold or Yellow. But I don't see matches for C3DA - B, C0DD - C, C1DD - A, or C4OA - A. Maybe I'm missing something?
I only see two matches: C0DD - A is Wine colored, and C0DD - B is Gold or Yellow. But I don't see matches for C3DA - B, C0DD - C, C1DD - A, or C4OA - A. You're looking in the wrong place for the part numbers.*
Ok, I was obviously looking at the wrong #'s. Now that I see what the part #'s are I've got matches on all but CODD 17271-C. Am I missing it?
And, help me understand this, please:
* And btw: If you read thru the DRIVEN gear applications, you will see #2 gears also. #2 gears in truck catalog listed as #3 gears in car parts catalog.
Gary I looked through the pages and I get what you are saying. I don't have a problem differentiating between the pages and understanding what each one is for.
Also, looking at the “Driven Gear” page I see you highlighted the correct column to let us know which one to look at. I think the only way you could make it any easier would be clearing that “Drive Gear” column.
Chris - Thanks. That's the best review so far. But I think I'll pass on clearing out the drive gear column. I can do that, and have done it in some cases, but in this case it would be extremely tedious. I have to use the eraser tool in something like Paint to carefully clean out the box while leaving the lines. All the way down for each gear ratio. So if it works for you I think I'll just leave it.
All - I need some guidance on what would be most helpful to you. It appears that I have the ability to embed a spreadsheet on my page so you could play what-if games regarding speedo gears. So I need to understand what your needs might be. I think these two approaches will suffice, but you tell me:
Simple: In this approach all you do is input your current driven gear size and your indicated speed at an actual 60 MPH and it will tell you the indicated speed using the other 5 driven gear sizes. This assumes you are keeping tire size, axle ratio, and drive gears the same.
Thorough: This approach will require that you tell it the "old" and "new" tire size and rear axle ratio, plus the current drive and driven gears, and it will show you the indicated speed and/or error with each of the 36 gearing combinations. In other words, you can change any variable and find out what your options are.
Do these two approaches look like they would answer your possible questions?
The driven gears range from 16 teeth to 21 teeth and the drive gears range from
6 to 8 teeth. This has been Ford's system since the 1960s and if you look at the
part numbers a most all are C prefix.
On my 1981 F100 that was stock with a 302 and AOD transmision and 3.25 rear gears, the transmission has a 7 tooth drive gear and when I changed the rear end
ratio to a 3.70 ratio I had to get an aftermarket 23 tooth driven gear to get the
correct speed reading with that ratio.
I got it fron Baugmann Engineering in Clemson, SC if anyone needs one.
Actually, the drive gears go to 9 teeth in the form of D7TZ 17285-A, as shown here: Speedometer Drive Gears - Gary's Garagemahal. It was used on many vehicles with 2.47 to 3.00 gear ratios.
Anyway, it is good to know that aftermarket sources have different driven gears so there are options as it is much easier to swap out the driven gear than the drive gear.
04-29-2016 | 01:19 PM
However, if y'all think it is confusing please tell me as I could meld the two pages, Drive and Driven, into one longer page and leave out the year-by-year Drive Gear tables. On the other hand, it does provide a different, and in some ways easier, way to look up the Drive Gear #'s. 
But I think I'll pass on clearing out the drive gear column. I can do that, and have done it in some cases, but in this case it would be extremely tedious. I have to use the eraser tool in something like Paint to carefully clean out the box while leaving the lines. All the way down for each gear ratio. 
So if it works for you I think I'll just leave it. 