Gear Ratio
(RPM x Tire Diameter) / (Diff Ratio x Trans Ratio x Transf Case Ratio x 336) = MPH

Crawl Ratio
Trans Ratio x Transf Case Ratio x Axle ratio = Crawl Ratio

P-Metric to Inch Diameter
[(Section Width x Aspect Ratio x 2) / 25.4)] + Rim Diameter = Tire Diameter

Ex 235/75R15

[(235 x 0.75 x 2) / 25.4)] + 15 = 28.87"

Tire Change (Find a new Gear ratio)
(New Tire Dia / Old Tire Dia) x Old Axle Ratio = New Gear Ratio

Speedometer Adjuster (With Oversize Tires)
(New Tire Diameter / Old Tire Diameter) x Speedometer MPH = Actual MPH

Another Great Gear Ratio Calculator
The below table can be used to get a rough idea on gear ratios. The colors represent ideal RPM's at highway speeds (65). For highway cruising and best fuel economy stay towards the yellow (2600 rpm), around town daily driving is color coded green (2800 rpm), and for better towing power or just more 4-low power use the ratios near the red (3100 rpm). These calculations are assuming a manual transmission with a 1:1 ratio. If you drive an automatic your RPMs will be slightly higher, and the opposite is true if you have overdrive (your RPMs will be slightly lower).

Chart and info from Mark I. Medina www.4Lo.com