I'm looking for anyone who can tell me a mathematical way to figure what speedo gear I need to fix my speedo. I went from 31" tires to 33" tires, (255/75R16 to 305/70R16). Then I went from 3.08 gears to 4.10 gears. My original speedo gear has 19 teeth on it! Someone told me I needed a 21. I have the 21 on it now, and it seems closer, but at highway speeds I can tell its off. Can someone explain, and tell me what I need?

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The tire switch is going to increase your overall gearing, while the gears reduce it. It is a simple matter of ratios:
Your old tires were 31.1" and the new ones are 32.8" (approximately). The new gears are 4.10 and the old ones 3.08. Therefore:

19*(31.1/32.8)*(4.1/3.08) = 23.9 or 24 teeth - but I heard somewhere that the largest gear was 23 teeth, so that may be the best you can do.

Ian

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99 Expedition EB
5.4l, 4x4, 4.10 LS
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