You need to know your tire height, or tire diameter, in order to work with RPM calculations relevant to gear ratio. Use this handy calculator to determine tire height.
Tire specifications usually follow a three number format, such as 245/75R16, where 245 is the WIDTH of the tire tread in millimeters, 75 is the ASPECT (or percentage of tread width to yield the sidewall height), and 16 is the WHEEL diameter. Using the numbers in this example, you'll get a tire height of 30.5".
The calculated value should be used only as a guideline for RPM calculations. Quite often, if you measure your tire diameter from the ground to the top of the tire, it measures less than the calculated value. It may "round up" closer to the calculated value while in motion. Tires with the same specification but made by different manufacturers may also have different actual diameters.
Truck and off road tires may have a specification like 30x9.5R15, where 30 is the tire diameter, 9.50 is the tread width and 15 is the wheel diameter. You don't need the tire height calculator in this case.
Gear Ratio Calculator
Enter the number of teeth on your gear set to find the gear ratio.
Example: Ring Gear / Pinion Gear = Ratio
Use this calculator to determine what RPM your engine will run based on gear ratio, tire height, transmission and MPH. You can try various combinations to come up with an appropriate combination of gear ratio and tire height for you vehicle. The RPM calculator should be used as a guideline for comparative purposes only. Your vehicle will probably run at different RPM's than those you calculate.