Speedo Calabration

My 1981 K-20 had 9.50 x 16.5 tires that were 30" tall I replaced them with BFG 12.50 x 16.5 that are 33" tall I used SnoMan's web site to see how far the speedo was off
at 85 it was 93.5 see http://www.snoman.com/HTML/axlecalc_5a.html witch is A great site Thanks SnoMan I know that I can replace the speedo gear in the tranny this is A simple swap any ideas on the gear size or GM part #
Thanks Dave Las Vegas NV.
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Dave wrote:

Try using two less teeth on the driven gear (one on the cable) and that should put you real close. Also for what it is worth, if you plan to regear sometime, one step deeper with current spedo gear would about be dead on too. Gears are color coded and I have about a half dozen of them here somewhere that I will have to dig out sometime and list the on my site by color and tooth count.
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TheSnoMan wrote:

    You know, that is something the vast majority of people don't know. Even people that work in drive line & speedometer calibration shops. Heck even when switching from one GM gear ratio to another the countermen don't know what speedometer drive gears go with what tire size and rear axle ratio. Charles
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K series truck, Drive gears can be a little expensive. They are in the t case. Driven gears are exactly the same as 2wd models. Make sure you replace all the orings and gaskets properly. A small leak in the t case might go unoticed until it is too late. You should have a NP208. Read the red tag on the t case to see what kind of lube you need. I think it will say tranny fluid instead of gear oil. But make sure you check. I'm not 100% sure. Either way use a good synthetic lube. t cases seem to never get serviced properly and a few extra $'s in lube will extend your service intervals.

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I have changed them a LOT and never changed the first O ring. If it is not leaking before you change it, it should be fine afterwards. Also no need to use SYN fluid (it takes ATF), just chage it every 30K or so and check the level with every oil change. If you run in water a lot you what to check and change it often and here SYN is a waste because contaminated fluid is bad whether it is SYN or dino oil.
David Johnson wrote:

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Dave wrote:

Borrow a Garmin GPS & read your speedometer every 10 mph on a flat road
vs. the true GPS speed... GPS reading is accurate to 0.1 mph...
This will put you in the ballpark as to the gear change needed...
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