Gearing Calculator

This gearing calculator is designed to calculate the gearing of a 4x4 drive train. This information can be used when designing modifications to the drive train of your 4x4, but it can be used for most vehicles. This calculator will work with any configuration I have ever seen. Up to six transmission ratios can be entered - which allows the use of a six-speed transmission. Two auxiliary gearing units can be entered because it is possible that someone will put an overdrive or underdrive in front of a BW1339 Quadratrack transfer case with a Milemarker fixed overdrive. Two transfer case low range ratios can be entered because some rock crawlers use two transfer cases to achieve low crawl ratios.

Note: While the program calculates everything correctly for up to four auxiliary gear boxes, the results for only two will be displayed in the bottom grid (two auxiliary gearboxes will yield four ratios). If you plan on having more than two "range" levers in your vehicle, you will need to use the [Create Results Window] pushbutton in order to view all of the low range and auxiliary gear combinations.

Auxiliary Gearing Units (up to two allowed).
Enter any overdrive or underdrive units. Examples include the gearboxes made by Advanced Adapters, the Saturn overdrive unit that can be installed on the Spicer 18 transfer case, and the Milemarker overdrive that can be installed in the BW1339 Quadratrack transfer case. Enter the gearing as a ratio (ex: enter "0.76" for a 0.76:1 overdrive or "3.35" for a 3.35:1 underdrive). Alternately, an overdrive ratio can be entered as a percentage (ex: enter "16%" for a 16% overdrive unit). Check the [Can be Switched] box if the unit can be switched (there is a lever). Leave it blank for a permanent overdrive (like the Milemarker kit in the BW1339 Quadratrack transfer case).

Transfer Cases (up to two allowed).
Enter the gear ratio of the low range unit in your transfer case(s).

Axle Ratio.
Enter the gear ratio of your axles.

Tire Diameter.
Enter the diameter of your tires. Since tires ratings are seldom accurate, we suggest that you measure the diameter from the top of your tire to the ground. Since the bottom of the tire is slightly flat, it will travel slightly less distance per revolution than might be expected. The diameter of the tire measured from top to bottom is a good compromise.

Press [Calculate] to display results in the bottom part of the window. Press [Create Results Window] to display results in the bottom part of the window and create a new window with the results.

Note: The results can be cut and pasted into a mime-compliant email.

While the crawl ratio is a nice "gee whiz" number that is useful for bragging rights, the actual speed of the vehicle at various engine speeds is more useful information. A larger tire will require a lower crawl ratio to accomplish the same "crawl speed".

Once you are off the trail, it is desirable to keep up with the freeway traffic, get better than three or four miles per gallon, and not thrash your engine. Since I have found (by experience) that a Jeep Wagoneer is perfectly stable at a speed of 100 miles per hour (making it a very versatile vehicle), I have included information about speeds of up to 100 MPH.

This section displays the overall ratio (not including the axle ratio) for each gear. It also displays the vehicle speed at 500 and at 5000 RPM. This gives you an idea about the speed range for each gear, and for each range/auxiliary setting.

Note: Only four range/auxiliary ratios are displayed. If you have more than two range shifters (which will produce four ratios), press the [Create Results Window] pushbutton above.