You will need to know your over all tire diameter in inches to use this calculator.  

If you have metric size tires (225/75/15) you can convert them to inches by using the 'Metric Tire Size Conversion To Inches'.  

This calculator will take your gear ratio and tire sizes and tell you what the effective or Actual Ratio is.  It will then suggest a New Ratio along with a Hi and Low alternative.  This New Ratio will return the vehicle to it's original performance after the addition of larger tires.  Obviously, you will have to round the new ratio to the closest available ratio.