I was sufing the web and found this article.

If you're running a tall tire (30"+), then you need to take that into account in your gear selection. That 3.50 gear and a 27" tire might be the equivalent of a 3.00 gear with a much taller tire. Figure what RPMs you can live with at highway speed, and then figure out what gears will put you there with your tire and tranny selection (you might have to go with 4.56's to get you there with a tall tire).

rpm = (mph x rear gear ratio x 336) / drive tire diameter.

Yes, a taller tire will effectively make your gear ratio act as if it were higher (numerically lower).

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[This message has been edited by Ratmotr (edited 03-29-2002).]