Ok. So I know my speedo is still off with my mods. So here is what I did. I have 33's instead of the stock 31's. And I changed my gears from 3.08 to 4.10. I was told to get a 21 tooth gear to replace my 19 tooth gear. That helped, but did not fix it. So decided to do my own math.

I take the new tire size divided by the old tire size. 33/31 = 1.064516 and

divide the original # of speedo 19 by that # and I get the effective # of teeth. 17.8.

Then I divide the new gear ratio by the old one 4.10/3.08 = 1.331168831 and

multiply that times the effective speedo tooth # 17.8.

By that, the number of new teeth I need to correct the speedo is 23.759 teeth.

I will probably just go with a 23 tooth gear. Can someone check my math and logic and tell me if this makes any sense?!?

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I agree:

19 *(31/33)*(4.1/3.08) = 23.76

You might want to go to a 24 tooth (if it is available - someone told me 23 was the largest) because it will still be off by +0.76/23 = + 3.3% as opposed to
-0.24/24 = -1%.

Good Luck

Ian


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99 Expedition EB
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