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Note that in the latest version of the gear ratio calculator, there is no calcualte button. Every time you make a selection, the form will automatically update. If you don't get any numbers at the bottom of the screen, make sure there is information filled in for the whole form, including tire size. If any information is missing or any fields are filled with text that isn't a number, the form will be left empty.
First thing to do is select SAE or metric. SAE measurements are done in feet and miles. Metric measurements are done in meters and kilometers.
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Step 1: Select transmission. You can select a transmission from the drop down list. If your transmission is not in the list but you know the gear ratios, you can select "Enter your own numbers" and put those ratios into the form. Be sure to put in the number of gears in the transmission (excluding reverse but including any overdrive(s)). Note that in the drop down list, some transmissions have more than one set of gears available. If you select anything other than "Enter your own numbers" you will not be able to change any of the numbers. But if you change back to "Enter your own numbers" the fields will be filled with the data from the last transmission you had selected so you can modify it as you see fit.
It is important to note that the torque converter in automatic transmissions allows substantial slippage at low speeds. This varies by the weight of the vehicle, the incline, obstacles the tire is trying to go over, stall speed of the torque converter, power of the engine, etc. etc. There is no way to estimate this without entering way too much information. A good rule of thumb to use when estimating actual speeds and RPMs when crawling is to double the RPMs or cut the speed in half. That will get you in the right ballpark.
Step 2: Select transfer case. If you don't have one, leave the selection at "None". As with transmissions, you can select one from the drop down list or enter your own numbers. The same behavior applies. If you select one, you won't be able to modify the numbers but you can switch back to "Enter your own numbers" and the fields will be filled with the last entry you selected and you can modify them all you want. Note that some transfer cases were offered with more than one low range ratio. Also, transfer cases with aftermarket low range gears are covered in the list with the stock transfer cases. If you are using the Atlas 4 speed transfer case, select one of the noted selections in the list and select the "Atlas 4 Speed range box" entry in the underdrive section. If you select a STaK 3 speed transfer case, the third colulmn in the ratio display will show the extra ratio. If you enter your own numbers and put anything in the STaK ratio entry, the form will try to use it.
Step 3: Select underdrive. Leave as None if no underdrive is used. You can put in your own numbers if you have fabricated your own underdrive based on the low range of another t-case. If you have an Atlas 4 speed, use the "Atlas 4 speed range box" field to factor in the built in reduction unit that makes it a 4 speed. The selection for the 203/205 doubler is for the 203 reduction unit to go with the 205 you should select above.
Step 4: Choose axle gear ratio. Type in the ratio of your axle gears. This form will accept any number that fits in the field.
Step 5: Select tire size. Metric sizes are listed after inch sizes. This form automatically figures out tire diameter based on metric size so you don't need to do that before hand. If you select inch sizes you won't be able to modify the metric drop downs. If you select metric tire sizes, you won't be able to modify the inch tire size. Keep in mind that advertized tire size can be off by an inch or more from actual tire size. This does affect the calculations so if you want a more exact size, measure your tire and enter it in the inch size. Note that the calculations are based on the fully inflated tire size. If you run at very low pressure, your effective tire size is reduced. To get a good approximation of effective tire size, measure from the ground to the center of the tire when at trail pressure. Double that result and enter it in the form.
Step 6: Customize your RPMs and Speeds in the form. You can select the RPMs and Speeds at the top of each column in the forms below. The default values are set up for the average domestic Jeep or 4x4 pickup with a gas engine. It is also set up for SAE measurements rather than metric. So when you switch to metric you may want to change the speeds in the bottom section.