73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Interior & Equipment => Instrumentation => Topic started by: gntblue on November 08, 2017, 02:18:14 PM
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I have a 1974 K-10 4x4, 350 w/ TH350 w/ the part time conversion.
Can someone point me in the direction of a website or document that shows the gear calculations for speedometers? All I can find is conversion info for the th350 not out of the NP 203 I recently installed a new Electronic Auto meter speedometer and their electric sending unit. Since this change the speedo is reading about 20 mph fast. I removed the reduction box that was on the end of the old cable so I'm guessing I will need to switch out the driven gear.
Here is what I have:
31" x 10.5" x 15 Rim
3.07 Gears
Drive gear on output shaft is red
Driven gear is steel (18 tooth)
Also info on places to order the new driven gears would be great too.
Thanks
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I think NOVAK conversions company carries those speedo gears. Another place that may be able to help is Allstate Gear.
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Usually what they did with the NP203 in production is leave the drive gear and driven gear alone and put a Bendix gear reduction unit on it. That was in production, and the service/parts manuals include the parts numbers for those units based on the ratio.
What you need is a solid read on indicated speed at a given known speed. You can get this by running 10 miles on the interstate between mileposts and comparing to the odometer reading difference.
Once you have that ratio, you get the correct ratio adapter.