64 C10 - gets a T5 transmission

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Originally Posted by Lugnutz65

Longbed and shortbed should take the same size cable. I bought my new cable from NAPA speedo cable BK 615-1606 which is 87" but if you can find an 84" cable that would be best. My 87" cable is plenty long.

Inside the T5 you have a drive gear that is either blue or red. See photo.
Use THIS ONLINE CALCULATOR and use the section named SPEEDOMETER GEAR CALCULATOR to determine what driven gear you need. You will need to know what your rear tire diameter is, the rear axle ratio (diff gear), and the number of drive gear teeth in your T5. Blue has 7 teeth and red has 9 teeth.

Driven gears are available new from vendors (HERE IS ONE) but are only available in 18 teeth through 22 teeth. If the online calculator shows you need a driven gear that's no available, please post your info and maybe there is another solution, such as a speedo ratio reducer. (see picture)

How do I find the rear axle ratio (diff gear)? Did all 1964 C10 with 235 motor and 3 on the tree have the same ratio? Can I find out by propping the rear wheels and counting the driveshaft revolutions and tire revolutions?

How do I get the drive gear out of the T5 to check?

There is a radar speed trailer near me that tells your speed on a display. When it says I'm doing 30, my speedo says 40. The spread is higher with higher speeds, like 35 actual speed vs 58 on my speedo.
I was hoping to use these numbers to calculate the percentage change I need and get the correct gears.