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PSOM, Tires, Gears & RPM Calculator - User Guide

User Guide


Introduction

The PSOM, Tires, Gears & RPM calculator PSOM, Tires, Gears & RPM calculator PSOM, Tires, Gears & RPM calculator can be used to calculate the PSOM conversion constant for a particular tire size, and also calculate RPM for tire size, transmission, transfer case and differential gear ratios.


PSOM Calculator

Enter one of the 4 values to solve for the other 3 values.  Calculation occurs upon the changing any one of the values.  Use the tab key or mouse to move between value fields.  Although tire size in diameter can be used, it is more accurate to use tire revolutions per mile, rolling circumference or rolling radius, respectively in that order.

WARNING: The formula for calculating the PSOM conversion constant may be vehicle and/or manufacture specific.  The only formula currently supported is for the Ford Bronco and was obtained from PSOM Conversion Constant formula for Big Broncos at off-road.com (Tire Revolutions Per Mile x 108 / 8000).  If you know of others I'd appreciate your letting me know so they can be included.

WARNING: The number of times a  PSOM can be programmed may be finite.  For instance the '93 - '96 Ford Bronco PSOM's can only be programmed 6 times and the '92 only 3 times.

WARNING: The PSOM, Tires, Gears & RPM calculator is intended only as an aid in the comparison and narrowing in on a range of desirable operating factors and conditions and should not be solely relied upon for determining any of the values calculated.  Prior to making any changes or purchases, selections should be calculated by hand using appropriate formulas.

Calibration is performed by programming the conversion constant into the PSOM.


Gear Ratio & RPM Calculator

The Gear Ratio & RPM calculator can be use to solve for the engine RPM, vehicle MPH, transmission gear ratio, transfer case gear ratio, differential gear ratio or tire size.  The overall ratio is always calculated.  Note that the "Overall Ratio" calculation is based on transmission, transfer case and differential gear ratios only and does not include torque converter slippage.

To solve for one of the six values select the 'radio' button next to the value you wish to solve for and enter the other five values.

The gear ratios for some of the more common transmissions and transfer cases can be easily entered by selecting them from the drop down lists and selecting a particular gear or range.  e.g. 1st, 2nd, D, OD and Hi, Lo.  For gear ratios of transmissions and transfer cases not listed, select 'Custom' and enter the values desired for each gear or range.

In addition, torque converter slip can be adjusted for by entering the percent of slippage.  Note that the "Overall Ratio" calculation is based on transmission, transfer case and differential gear ratios only and does not include torque converter slippage.

MPH and RPM can also be conveniently entered/changed by clicking on the desired value in the MPH / RPM table.  This is most useful when solving for gear ratios or tire size.

For a list of transmissions and transfer case gear ratios click the "Gear Ratios" button.

Note: Tire revolutions per mile from the PSOM conversion constant calculation is used for calculating engine RPM and vehicle MPH.  If that value is empty the engine RPM and vehicle MPH can not be solved.  One of the four variations of tire size must be entered and the other three will be calculated, as well as the PSOM conversion constant.  Although tire size in diameter can be used, it is more accurate to use tire revolutions per mile, rolling circumference or rolling radius, respectively in that order.

WARNING: The PSOM, Tires, Gears & RPM calculator is intended only as an aid in the comparison and narrowing in on a range of desirable operating factors and conditions and should not be solely relied upon for determining any of the values calculated.  Prior to making any changes or purchases, selections should be calculated by hand using appropriate formulas.


Special Buttons

Show A Friend Button - Sends URL to a friend with your current values.  When they open the URL the PSOM, Tires, Gears & RPM calculator will automatically be loaded with your values.  Very handy for sharing information when working with others to find the "right" setup.

Gear Ratios Button - Provides links to other resources for transmission and transfer case gear ratios.

User Guide Button - Opens the user guide document (this document).


Glossary

Revolutions Per Mile
Number of tire revolutions per mile, with normal load and tire inflation.
Some manufactures provide revolutions per mile information for their tires in brochures and on their web sites.

Rolling Circumference
Distance in inches traveled by 1 tire revolution, with normal load and tire inflation.
Perhaps the easiest way to determine the rolling circumference of your tire is to mark it with a substance that will be transferred to the surface as it rolls, then measure the distance between the marks.  Load and tire inflation can affect the rolling circumference considerably.

Rolling Radius
Distance in inches from ground to center of wheel, with normal load and tire inflation.

PSOM Conversion Constant
A 3 to 4 digit (decimal point omitted) value to calibrate the PSOM (Programmable Speedometer & Odometer Module) to tire size.

WARNING: The formula for calculating this value may be vehicle and/or manufacture specific.  The only formula currently supported is for the Ford Bronco and was obtained from PSOM Conversion Constant formula for Big Broncos at off-road.com (Tire Revolutions Per Mile x 108 / 8000).  If you know of others I'd appreciate your letting me know so they can be included.

WARNING: The number of times a  PSOM can be programmed may be finite.  For instance the '93 - '96 Ford Bronco PSOM's can only be programmed 6 times and the '92 only 3 times.

RPM Sweet Spot
The Yellow, Green & Red designations in the MPH RPM table indicate a range of operation for Better Fuel Economy, Sweet Spot & More Power.

RPM Sweet Spot Legend

Better Fuel Economy
Sweet Spot
More Power

NOTE: The ranges of these indicators are arbitrary and are not scientifically derived.

PSOM, Tires, Gears & RPM Calculator    Forditude™
Copyright © 2002, Alvey H. Stubblefield.  All rights reserved.

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