Differential Gear Ratio Calculators
Our Differential Gear Ratio Calculator can help you select the correct gear ratio to maximize the performance of your vehicle. The addition of larger tires can greatly decrease performance and fuel economy. Choosing the right differential gear ratio will help regain lost power and torque.
- The Tire Diameter Calculator will allow you to determine your actual tire diameter.
- The RPM Calculator will give you an estimate of your engine RPM, based on tire diameter and the gear ratio you select.
- The Gear Ratio Calculator will determine the ratio of a gear set, based on the number of teeth on the ring gear and teeth on the pinion gear. This is especially helpful when trying to identify a gearset that needs replacement with the same ratio ring and pinion.
Tire Diameter Calculator
START HERE! You need to know your tire diameter in order to work with the RPM calculator relevant to gear ratio. Inaccurate tire size input will greatly affect the results from the Gear Ratio Calculator.
Tire specifications usually follow a three number format, such as 275/75R16, where 275 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 32.24″ by using the calculator below.
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.
Engine 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.
Recommended Engine RPM @ Highway Speed
- 4 cylinder: 2200 – 3200
- V6 cylinder: 2000 – 3200
- Small block: 1800 – 2800
- Big block: 1800 – 2600
- Diesel: 1600-2800
For a complete listing of the Ring & Pinion Gear Ratios available for your application, go to the Sierra Gear Ring & Pinion Gear Sets page.
Gear Ratio Calculator
Identify the Ring and Pinion Gear Ratio you currently have by counting the teeth on the Ring and Pinion gears. Enter the number of teeth on your ring and pinion gear set in the calculator below to find the gear ratio, or simply divide Ring Gear tooth count by Pinion Gear tooth count (Example: Ring Gear / Pinion Gear = Ratio)
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.