700r4 speedometer adjustment after gear swap help

Swearbody

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First sorry for my absence from the forum as of late. I have stage 3 cancer and its been rough. **** Cancer
That being said
I did finally get the Gears and Diff done. Went from 2.7x to 3.90 and now have a eaton truetrac. I just drove it for the first heat cycle and well damn...I didn't anticipate the speedometer being off. I have no clue how to fix this. I expect to find that there is an easy fix, like a gear reduction for the speedo gear or something along that line...however I at this time have not had the energy to do the research. I know I normally would just search first and then ask but im hoping you'll forgive me this time. Any info on the matter will be appreciated.
 

Swearbody

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IM basically just asking if anyone knows the drive and driven gears size. Im coming up with 15 and 41 for speedo gears. Thats having come from a 2.73 to a 3.90. I think I may also need a larger housing/sleeve for the drive gear on the output shaft too. So basically 15 driven 41 drive would be an exact ratio match with a 28.74 tire.
 

Matt69olds

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https://www.tciauto.com/speedometer-gear-calculator

You need to know exact tire diameter,gear ratio, and ideally either the drive or driven gear tooth count. Play around with entering different gear tooth combinations until you get a drive/driven gear that is as close to a whole number as possible.

For example, if it says you need a driven gear with 33.7 teeth, you could go with either a 33, or 34 teeth. Your speedo will be slightly more accurate with 34 than a 33.
 

Swearbody

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Thanks Matt!! Thats the method and calculator I used to determine the 15-41 combo. I cant imagine why I didnt realize the speedo would change with a gear change. Thankfully its an "easy" fix,LOL.
 

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Thanks Matt!! Thats the method and calculator I used to determine the 15-41 combo. I cant imagine why I didnt realize the speedo would change with a gear change. Thankfully its an "easy" fix,LOL.
It doesn't change with a gear change if your tire size remains the same.
 

Matt69olds

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How do you figure changing final drive gears won’t affect speedometer accuracy?
 

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