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Our Facility Differential Gear Calculators
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 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″.
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.
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
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
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Featured Item
AEV ProCal Module
The ProCal Module is an easy way for Jeep Wrangler JK owners to correct the speedometer and shift points when larger tires are installed and gear ratios have been changed.
The Procal Module provides functions for large tire recalibration, axle gear ratio recalibration, tire pressure monitor recalibration, daytime running lamp activation/deactivation, one -touch turn signal activation-deactivation, engine idle speed options and precise steering wheel centering. These functions are available for all JK Wranglers including the new 2012 models. Owners of earlier versions of the ProCal can send their module in for a re-flash, updating software for use with 2012 model JK's.
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