You don’t need the tire diameter calculator in this case.Ĭalculating a 'Return to Stock' Gear RatioĪfter installing larger tires, it is advantageous to change the gear ratio to regain lost acceleration and fuel economy. 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. Using the numbers in this example, you’ll get a tire height of 32.24″ by using the calculator below. 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. If you do not have access to measure the actual tire height, use the calculator to determine tire height based on the manufacturer’s size specifications. Tires with identical size specifications that are made by different manufacturers may also have different actual diameters. The most accurate way to determine tire height is to physically measure from the ground to the top of the actual tire. This dimension will “round up” slightly when the vehicle is in motion. The calculated value should be used only as a guideline for RPM calculations. Inaccurate tire size input will greatly affect the results from the Gear Ratio Calculator. START HERE! You need to know your tire diameter in order to work with the RPM calculator relevant to gear ratio.

O2j gear ratio calculator