Gear Ratio Calculator

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This form allows you to calculate final drive ratios as well as see a comparison of speeds and RPMs within operating ranges of the vehicle. This calculator is useful for planning your rig, allowing you to see what kind of performance to expect from different combinations.

Disclaimer: This form is only to be used for estimation purposes. Exact results of combining equipment may vary from the estimates provided in this form. The data provided here is done so as-is with no warranty expressed or implied.

Note: This calculator is continually being updated. The lists of available equipment will probably never be complete. If you find that the parts you would like to use are not yet listed, please send the information to me at zerocool@grimmjeeper.com and I will do my best to include it as quickly as possible.

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. Also, keep in mind 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. You can select a transfer case from the list or enter your own ratios. Most transfer cases have a 1.0 high range. 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" entry in the underdrive section. If you select a STaK 3 speed transfer case, the underdrive selection will be ignored and the 3rd speed will be listed in the left column of the underdrive section.

Step 3: Select underdrive. Leave as None if no underdrive is used. If you have an Atlas 4 speed, use this 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: Select axle gear ratio. Use closest approximation if your exact ratio is not found, i.e. use 4.10 for 4.11 and 4.09 gears. The difference will not have a significant effect on the results of the calculations.

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. 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 select it from the closest 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 select that from the list.

Step 6: 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. You may not be interested in high speed driving if you drive a dedicated rock crawler or you may want to estmiate for higher than average speeds for Baja style racing. You may have a high revving engine, such as you would find in a Suzuki Samurai or a diesel engine with a 3500 RPM redline. So select the ranges you are interested in.

Step 7: Calculate

There are two copies of the form so that you can do a side-by-side comparison of two combinations.

Step 1: Select transmission from the drop down list or, if it's not in the list and you know the gear ratios of your transmission, enter them below.

Number of forward gears
1st Gear
2nd Gear
3rd Gear
4th Gear
5th Gear
6th Gear
Reverse
Manual or Auto with lockup torque converter
Automatic without lockup toque converter

Step 2: Select transfer case from the drop down list or, if it's not in the list and you know the gear ratios of your transfer case, enter them below.

High Range
Low Range
STaK 3rd Range

Step 3: Select underdrive.

Step 4: Select axle gear ratio.

Step 5: Select tire size.

inch
Metric /R

Step 6: Hit the calculate button and read the results below.

If you want to clear the results and try again, hit this button

The following chart lists the final drive ratio of all combined gears (transmission, transfer case, underdrive, axle) in all possible combinations

Final Drive Ratio
Gear Underdrive High Range Underdrive Low Range
Transfer Case High Range Transfer Case Low Range Transfer Case High Range Transfer Case Low Range
1
2
3
4
5
6
Reverse

The following chart lists the crawl speed you will be going while the engine is at a given RPM. Crawl speed is calculated based on the lowest low range (transfer case and underdrive in low range) available in your rig.

Crawl Speed at Given RPM in Feet Per Minute
Gear RPM RPM RPM
1
2
3
4
5
6
Reverse

The following chart lists the road speed you will be going while the engine is at a given RPM. Road speed is calculated based on the transfer case and underdrive being in high range.

Road Speed at Given RPM in Miles Per Hour
Gear RPM RPM RPM
1
2
3
4
5
6
Reverse

The following chart lists the RPMs your engine will be turning while driving at a given speed on the road.

Road RPMs at Given Speed
Gear MPH MPH MPH
1
2
3
4
5
6
Reverse

Step 1: Select transmission from the drop down list or, if it's not in the list and you know the gear ratios of your transmission, enter them below.