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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: LTZ C20 on August 16, 2014, 01:06:29 am
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Hey all, have a quick question here. New 700R4 in my 73 2wd 3/4 ton. Trans builder installed RED drive and driven gears for the speedo. Speedo doesn't read proper, go figure haha. I found a site with a calculator to determine which gears were needed, using tire size and rear end gear ratio.
Here's the calculator:
http://700r4.com/speedoCalc/speedocalc.shtml (http://700r4.com/speedoCalc/speedocalc.shtml)
Tire size is 265 / 75 R 16. Rear gear ratio is 4 : 10.
Then I found another site with a chart to find the right color depending on tooth number. Here it is:
http://www.tciauto.com/tc/speedometer-gear-calculator/ (http://www.tciauto.com/tc/speedometer-gear-calculator/)
Using the calc, it says my DRIVE gear should be a 19 tooth, my DRIVEN gear should be a 34 tooth.
Using the chart, the DRIVE gear should be YELLOW and the DRIVEN gear should be LIGHT GREEN.
Does this sound right? I think it is but want to be sure. Not taking it apart twice. Thanks guys.
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To be honest, I have no idea if that sounds right or not, but I can tell you that the folks at TCI were very helpful to me with this same thing. Last year I bought one of their TH350 transmissions for my '76 to mate up with a new GM Crate Engine. My speedo was off after the swap, so I checked (as best I could) my rear axle ratio, and of course I knew my tire size. I called them up and they told me exactly what I needed. I ordered it, and it's pretty well dead on.
Someone else with more knowledge will hopefully chime in, but I just wanted to give TCI their props since you mentioned them above. :)
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Ok cool thanks. I will see about calling them next week and see what they say.
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Based on the info you provided, that does sound correct...
http://73-87.com/7387garage/drivetrain/speedo.htm