speedo help

[Destroyer000]

alright so iam at it again with messing with speedo gears.. i recently had 33s with a black 20 tooth speedo gear and a White internal gear which i believe is a 7tooh gear. my speedo was pretty closeit was off about a tenth of a mile.. well i just upgraded to 35s. iam running 35s with 4.10s and i jumped up my speedo gear to a 21 red tooth gear.. i followed a buddy from his place to mine and his speedo red 11.5 miles and mine was 8.5 so it 3 miles off. i just did the math with an equation that my gf told me and its off 26 miles for every hundred miles traveled. 100/11.5=8.6x3= 26 . i tired to get my speedo closer and swapped out the internal drive gear with a red one.. i believe its an 8 tooth??? iam having a hard time find what tooth it is.. i just did the math again and now my speedo is off 29 miles per every hundred. so what combination should be used????

according to this formula
driver gear x axle ratio x tire revoluion / 1000
8x4.10x599=19647.2/1000=19.6 a BLACK TOOTH

correct??







now back in the day when my truck was stock i had 3.73s 31s and a pink 19tooth gear. and a white 7 tooth internal gear using that formula it says it SHOULD have a 17.6 18tooth.. how is this accurate when it was a 19tooth stocK?

 

[Destroyer000]

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