TH-400 Speedo Drive Gear Replacement - Hot Rod Forum : Hotrodders Bulletin Board
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Old 03-15-2002, 01:53 PM
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Cool TH-400 Speedo Drive Gear Replacement

Has anyone replaced their speedo drive gear lately. I'm thinking about changing my drive and driven speedo gears to get the speedo reading correctly, but have never done this. Looks like the drive gear is fairly easily accessible after removing the tail section off the TH-400. I also understand that the factory drive gear may be nylon and subsequent replacements are steel and are secured on the shaft in different ways. Any advice would be appreciated. Transmission Novice (Scott)

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Old 03-15-2002, 05:13 PM
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Scott easy job all plastic gears small clip holds them on. Do you know what gears you need . How far are your gears off. There is a number cambo that you go by.Do you have a tach. I can tell you what you need. ROY
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Old 03-15-2002, 06:29 PM
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Hi Roy and thanks for the reply. I do have a tach and I do have the information on how far off I am. For example, when driving at 70 mph (based on elapsed time between mile markers) the speedo is only indicating 50. Do you have a formula to determine the correct gear pairing? Scott
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Old 03-15-2002, 10:40 PM
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How tall are your rear tires and at 60 on the speedo what does the tach say. ROY and almost for got what is the rear axle ratio.

[ March 16, 2002: Message edited by: Roy Newton- exprotech ]</p>
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Old 03-16-2002, 08:05 AM
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Roy, here is the information....P255/70R15 tires (29" diameter via tape measure). At 70 mph, 2300 RPM. 2.56 rear gears. Don't have an RPM reading at 60mph. Let me know if you need anything else. Scott
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Old 03-16-2002, 07:50 PM
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Scott my chart is for 60 MPH and at that speed with 29 tall tire and 256 gears you should turn 1775 RPM at 60 MPH.This is how you can tell how far you are off.ROY
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Old 03-17-2002, 03:03 AM
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Scott, Try this site out.
www.tciauto.com/tech_info/speedo_gears.htm


hope this helps
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