Gear Calculator

Tire Diameter Calculator

You need to know your tire diameter in order to work with the RPM calculator relevant to gear ratio.

Tire specifications usually follow a three number format, such as 275/75R16, where 275 is the WIDTH of the tire tread in millimetres, 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 32.24″.

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 identical specifications that are made by different manufacturers may also have different actual diameters.

Truck and off-road tires may have a specification like 30×9.5R15, where 30 is the tire diameter, 9.50 is the tread width and 15 is the wheel diameter. You don’t need the tire diameter calculator in this case.
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 identical specifications that are made by different manufacturers may also have different actual diameters.

Truck and off-road tires may have a specification like 30×9.5R15, where 30 is the tire diameter, 9.50 is the tread width and 15 is the wheel diameter. You don’t need the tire diameter calculator in this case.

Gear Ratio Calculator

Enter the number of teeth on your ring and pinion gear set to find the gear ratio. Example: Ring Gear / Pinion Gear = Ratio

RPM Calculator

Use the RPM calculator to determine what RPM your engine will run based on gear ratio, tire diameter, transmission and MPH. You can try various combinations to come up with an appropriate combination of ring and pinion gear ratio and tire diameter for your 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. See What gear ratio do I need in FAQ for a discussion on choosing a ratio.

RECOMMENDED ENGINE RPM @ HIGHWAY SPEED

  • 4 cylinder: 2200 – 3200
  • V6 cylinder: 2000 – 3200
  • Small block: 1800 – 2800
  • Big block: 1800 – 2600
  • Diesel: 1600-2800

Find the ratio you want? Contact the Differential Repair experts at Ring & Pinion Service at (916)635-9665 or use our online appointment application.