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Drivetrain, Gear Ratio, and Tire Calculators

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Welcome to our online calculators. Our calculators are very simple. These pages contain basic formulas, input fields, output values, and maybe a basic description, However we won't go into detailed explainations on the theory and basis behind the calculations, or functions here. However we may write some informational articles on them if there is interest, but these calculator pages are all business.

How to use the calculators: Enter your values in the boxes labeled above the calculation button. The calculations will appear in the boxes below the calculation button once the button is pressed, so the final calculated values are only correct when ALL your values are entered correctly.

Note: There is a known issue with older versions of Internet Explorer (IE8 and earlier) and the calculators. The issues are being looked into. We currently recommend that you update your browser if you are having issues.

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Tire Diameter

To calculate the tire diameter from the metric sizing we need to understand what the numbers are. Metric sizes start with LT or P (LT for light truck, P for passenger vehicle) and a number which is the section width measurement in millimeters, then a slash and another number which is the aspect ratio in percentage, and lastly an R followed by the final number which is the rim diameter size. In order to calculate the tire diameter from these measurements we use the following formula:

LT or P / R

Calculated Tire Diameter (inches):


Tire Size Change Calculator

Changing tire sizes will also affect various things such as your speedometer calibration and effective gear ratio due to the difference in tire circumference. In this calculator you can calculate the difference in the speedometer reading for a different tire size, as well as the effective gear ratio change, and the gear ratio needed to correct for the different tire size.

Enter your old tire diameter, new tire diameter, current axle gear ratio, and a current speedometer output. With this data you can see how the tire size change effects your gear ratio, the effective current gear ratio is your adjusted ratio if you keep your current axle gear ratio. The gear ratio needed for correction is the axle ratio you will need to install in order to correct your power output to equal the power output you had before the tire size change. The corrected actual speed field shows the actual vehicle speed when the speedometer reads whatever speed you entered into the 'speedometer output' field with the new tire diameter. And lastly is the percentage error that the speedometer shows due to the new tire diameter. Formulas used are:





Old Tire Diameter :

New Tire Diameter :

Current Gear Ratio :

Speedometer Output:

Effective Current Gear Ratio :

Gear Ratio Needed for Correction :

Corrected Actual Speed :

Speedometer Reading Error (precentage) :


Axle Gear Ratios

(under construction still)

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