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Through changing rear end gears and tire sizes, my speedo reads 60 mph at an actual 52 mph.

I want to buy a transmisison gear with the right number of teeth to correct the error. The current gear in the side of the transmission is colored orange. Were all these gears in all TH 400's the same basic dimensions, except for number of teeth? In other words, can I go to the dealer and say "I have an orange with X teeth. Give me a gear with Y teeth" and hope to get one that drops right in?

Based on needing a gear which will produce a 60/52 (15%) increase in teeth over the orange (how many ever that one has), anyone know what number of teeth or color I need so I don't have to try to explain to the counter guy?

Oh yeah, does it just pop out once the out side cover plate is removed?
 

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I've been through this several times and the easiest solution that I've come across is a speedo ratio adapter. The outfit that I've bought them from is called United Speedometer and Instrument. I don't have a number handy but they always advertise in Rod & Custom. They send out an information form which requires you to do a little work, you send or call in the info and they custom make you an adapter with the correct ratio adjustment. I have had no luck working with dealer parts men on this problem, they don't have any sort of chart to work from and getting the right part is trial and error at best. This can be expensive and very time consuming!
 
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Go to "resto shop" on this site then go to tech help. You'll find what you are looking for.........
 

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Each tooth will change your MPH by about 2.5-3.0 MPH. Early GM transmissions all use the same gears. Any dealer should have what you need. Get a gear with 4 more teeth and this should get you close. The gears are only about $8.00 so even if you don't get it close enough the first time you are not out too much and its still cheaper than a speedo shop.

------------------
Rick Schaefer
72 El Camino
Abilene Texas
 

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TH400 speedometer gear:
You didn't say how many teeth on the gear you now have. 400's use 2 factory adpters, one will handle 40 to 45 teeth, the other will handle 39 to 35 teeth. There are also two lenghts of these adpters. Make sure get the same lenght as your orig.
My 69 El Camino with 273 rear end has 38 teeth for the 400.
 

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Pulled out my old 1972 Olds Chassis manual and according to it the orange gear has 35 teeth. This of course assumes that Olds gears are the same as Chevy ones, which I believe they are. The 39 tooth gear is brown, and for fine tuning the 38 tooth is blue and the 40 tooth is black (but it will require a different sleeve).
 

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Discussion Starter #8
Thanks for all the input. I am in the embarrassing position of not knowing exactly what rear end gears are in it, I think they are 3.53's. Hence I can't use the chart.

If my orange is 35 teeth, then I need a 40.38 tooth gear. I'll settle for a black 40, but dumb question: what is the sleeve? And with a jump that big, does the mating "drive gear" have to be different to account for different pitch diameters, or anything like that?
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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Fred,

I pasted your URL into my browser and it bombed over at Geocities. Not Found. Might want to check it out.....

Tom
 

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The sleeve is the outer "flange" that the driven gear rides in. You will not need to change the drive gear inside the tranny; what the factory did if the change was too much was use a speedo ratio adapter like 68396 described. In my Olds Manual for a 3.42 rear axle gearing they used anywhere from a 39 tooth to a 42 tooth gear, depending on tire size and with a 16 tooth drive gear; all of these without a speedo ratio adapter. Sounds like you are in the ballpark.
 

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Is there such a think as a book or chart on how many gear teeth you need to run with what trans and rear end gear? I'm having the same problem.
 
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