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Speedo Gear Question
https://slantsix.org/forum/viewtopic.php?t=60935		 Page 1 of 2
Author:  Chuck [ Mon Mar 27, 2017 11:46 am ]
Post subject:  Speedo Gear Question
I am working on a '66 Valiant. I swapped in a later auto trans, probably from '69, and the speedometer is off about 12%. (I had to use the front clutch assembly from the '66 so the splines were correct for the TC.) I dug out some gears that I had, but their diameter is about 1 3/16 and the one in the trans is about 1 3/8. I didn't realize that there were different gear diameters for the later trannies. I don't know what my gears came out of. I acquired them sometime in the '80s when I had my shop.

I need to get a gear that will bring the speedo closer to the real speed. The current gear has 32 teeth, so I figured that if I added 12% to 32, I would get 35.84, so I would use a 36 tooth gear. However, when I looked up MoPar speedo gears at http://www.drivinithome.com/mopar-speedo-gear-chart and used the tire size (24") and axle ratio (3.23), it suggests 34 teeth. Gears are anywhere from $25 and up so I want to make the best guess possible.

Any suggestions?

Thanks,
Author:  slantzilla [ Mon Mar 27, 2017 12:40 pm ]
Post subject: 
Figure out what number you need and let me know. I have a bunch of them at the shop. I will be going down there this Sunday and will look.
Author:  emsvitil [ Mon Mar 27, 2017 3:21 pm ]
Post subject: 
I'd go with your calculations.........

Just remember that if the speedo is reading high, you need more teeth..... (and vice-versa)
Author:  SlantSixDan [ Mon Mar 27, 2017 3:59 pm ]
Post subject: 
Don't worry about speedo drive pinion diameter. The '66-up speedometer gear holder/adaptor can be installed in multiple positions, marked on the adaptor by tooth count ranges, to acommodate the different pinion diameters that go along with different tooth counts.

Are you real sure about your tire diameter? That's extremely small. It would suggest you're running miniature tires (175/80R13 for example).
Author:  Chuck [ Mon Mar 27, 2017 4:06 pm ]
Post subject: 
Quote:
Figure out what number you need and let me know. I have a bunch of them at the shop. I will be going down there this Sunday and will look.
Thanks! I'll let you know. I assume you have the 1 3/8 gears.

Would you have the early 19 teeth - PN 2204358/Blue for my '63?
Author:  Chuck [ Mon Mar 27, 2017 4:11 pm ]
Post subject: 
Quote:
Don't worry about speedo drive pinion diameter. The '66-up speedometer gear holder/adaptor can be installed in multiple positions, marked on the adaptor by tooth count ranges, to acommodate the different pinion diameters that go along with different tooth counts.

Are you real sure about your tire diameter? That's extremely small. It would suggest you're running miniature tires (175/80R13 for example).
I measured the tire diameter as best I could, but I didn't write down the tire size.

I am concerned about the gear diameter because I have two 32-tooth gears. One is 1 3/16 and the other is 1 3/8, so I don't figure they can both be used in the same tranny, and certainly not with the adapter in the same position. It's got me stumped.
Author:  DusterIdiot [ Mon Mar 27, 2017 4:21 pm ]
Post subject:  Wow...
Quote:
used the tire size (24") and axle ratio (3.23)
Taking a WAG that it's a 185/75R13 (23.93" diameter) and a 3.23... it computes out to a 35.4 tooth gear... so a 35 tooth with a little margin of error in the speedo reading...
Author:  emsvitil [ Mon Mar 27, 2017 4:30 pm ]
Post subject: 
Speedo's from the manufacturer tend to read high......

Don't forget that the rolling radius (or rolling diameter) will be smaller than measured.........
Author:  SlantSixDan [ Mon Mar 27, 2017 4:50 pm ]
Post subject: 
Quote:
I measured the tire diameter as best I could, but I didn't write down the tire size.
No good...you have to go by tire size when doing this calculation.
Quote:
I am concerned about the gear diameter because I have two 32-tooth gears. One is 1 3/16 and the other is 1 3/8
Interesting. Show us a pic? What part numbers are on the pinions?
Author:  slantzilla [ Mon Mar 27, 2017 6:52 pm ]
Post subject: 
I have all the '66 up gears. They are different diameters depending on tooth count. Just give me the tooth count you need and I will see if I have it. :D
Author:  Chuck [ Tue Mar 28, 2017 9:43 am ]
Post subject: 
I checked the gears again, and Claude got it wrong. The smaller diameter gears are 29 & 31 teeth, and the larger one is 32 teeth. Slantzilla, looks like I will need a 35 or 36 based on my calculations and DusterIdiot's.

I checked with Claude and the tire size is 185/70R14. Where do I look up the calculations you guys did?
Author:  slantzilla [ Tue Mar 28, 2017 10:59 am ]
Post subject: 
I will look Sunday and let you know Chuck. I think I have seen a 35 in my pile.
Author:  Chuck [ Tue Mar 28, 2017 11:06 am ]
Post subject: 
Quote:
I will look Sunday and let you know Chuck. I think I have seen a 35 in my pile.
My test numbers vary a lot, so if you could dig up a 34, 35 & 36, I'll try them all and send back what I don't use.

Thanks,
Author:  slantzilla [ Tue Mar 28, 2017 11:17 am ]
Post subject: 
Will do.
Author:  SlantSixDan [ Tue Mar 28, 2017 2:07 pm ]
Post subject: 
Quote:
I checked with Claude and the tire size is 185/70R14.
185 is the cross-section width of the tire in millimetres.
70 means the tire is 70% as high as it is wide.

So 185 × 0.7 = 129.5 mm, that's how tall the tire section is.

We have to double that figure, because the tire surrounds the wheel, so there are two sections to account for (the one at the top and the one at the bottom).
129.5 × 2 = 259

We divide that figure by 25.4 (number of millimetres in an inch)
259 ÷ 25.4 = 10.2

Add the wheel diameter (14" in this case)
14 + 10.2 = 24.2

Looks like you were pretty close. That's a really small-diameter tire to be running.
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