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I just bought a 4" lifted 06 X with 33" tires. its been at the shop since the prior owner ran all of the oil out of the rear differential and wacked the control arm so i've had new 4.10 gears put in, new lower control arms, new bushings and track bar. jsut got it back and feels like the speedo is off alot. i thought with bigger tires the speedo would read slower than actual speed but with a GPS i'm atcually going 10 miles per hour slower than the speedo says?? not sure if the prior owner put in a new speedo gear for 3.07 gears and 33" tires but, if he did would that be why i am going slower than the speedo says??
 

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I don't think the speedo gear has anything to do with gear ratios, but I could be wrong. I think it is proportional to tire size only. At 60mph your speedo will read 55.3mph if your gear is for a 30" tire. If it's for a 29" tire it will read 53.5mph and if its for a 27" tire it will read 49.6mph.
 

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My speedo chart says that for 33's with 3.07, you should have had a 26 tooth gear. Now, with the 4.10, you should have a 36 tooth gear in there.
 

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Here is some more help...
 

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Not wrong so much as just off the mark. It's all good. If you do not learn something everyday, then you are dead.

The speedo (and speedo gear) are reading revolutions at the output shaft. What the Heep does not know is how big your tires are and your gear ratios (in your diff). Now, if you change tranny gears, it would have no effect on the speedo.
 
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