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I just realized the other day that something is funny with my gearing. Until recently, I knew I had bigger than stock tires, and by using my GPS I knew my speedometer was off by about 20%. Fine.

Problem is, I was going to check my gas mileage the other day, and it hit me- its 20% in the wrong direction! Unless I'm thinking about this the wrong way, with bigger tires you should be going faster than your speedo says, but for me when the speedo reads 60 I'm only going 50. And I have 31" tires which I'm guessing is about a 10% increase from stock, so that makes me think I have about a 30% gear reduction somewhere... does this sound right?
 

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Your thinking right. Taller tires should make you read slower than your actually going. Going from a stock 205-70-15 (26.3") tire to a 31" tire should put you near 18% off. Either someone has done A, A t-case upgrade B, A ring gear swap. C, Your speedo is just way off. D. A combo of all the above. Your running at nearly a 40% reduction by the speedo readings.
 

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The speedometer drive comes from the output of the transfer case. Changes to the transfer case gearing will not affect the speedometer/odometer reading. Is it possible you have a 5.13 (Sidekick) or 5.38 (Yukon) Ring & Pinion installed?
 

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With the T-case gearing this is true it won't change the speedo reading. The best I can tell they offered two differant speedo gears for the end of the cable. One was a 19 tooth and another with 20 depending on year. Around a 10% change.
 
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