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My TJ came from the factory with 3.07 gears in the axles. But I've since replaced them with 4.56's. My stock AX-15 manual tranny has a first gear ratio of 3.83-to-1. And my NP231 transfer case turns 2.72-to-1 in low range. However, I just installed a TeraLow 4-to-1 in my transfer case. My crawl ratio originally started out at a pitiful 32-to-1. The axle gear swap, however, provided for a reasonable 47.5-to-1. And with the addition of my TeraLow, I now have a very respectable 70-to-1. Hey, wait just a minute!! Welcome to the This page covers a bunch of stuff I've learned about gears over the years (mostly from reading 4x4 magazines and on-line bulletin boards, and talking to other Jeepers). This is prolly one of the most complex aspects of building a trail rig, in my opinion. So pay close attention! Basically, gears effectively change the amount of torque produced by your engine by various factors called gear ratios. Suppose, for example, you didn't have any gears in your transmission; the tranny would be turning the same speed as your engine. It would be like trying to start rolling from a dead stop in 4th gear. You would probably stall the engine. Other gears, specifically those within your axles (or differentials), make it possible to transfer rotational motion from one plane to another (i.e., from the drive shafts to the axles). In other words, it makes it possible for your engine (which rotates in one direction) to push your Jeep by turning the wheels (which rotate in a perpendicular plane relative to the engine). A gear's ratio is the number of revolutions a small gear has to make, in order to make a larger gear complete one revolution. The lower your gear ratio, the more revolutions your small gear must turn in order to make the larger gear complete one revolution. Lower gear ratios have larger numbers (e.g., a gear ratio of 4.56-to-1 is lower than a ratio of 3.07-to-1). The lower your gear ratio, the more torque (or force) your machine will produce. This increased torque, however, will also increase the amount of stress placed on all the rest of your axle parts (bearings, axle shafts, and the axle housing itself). Lower gears will move your vehicle slower than higher gears, given the same engine RPMs (a good thing if you are building a rock crawler). Conversely, larger tires will push your vehicle faster, given the same gear size and engine RPM. Therefore, gear ratio and tire size go hand-in-hand. Use the table at the bottom of this page to help you determine the appropriate axle gear ratio for various tire sizes, depending on what type of performance you desire. In addition to tire size, there are several other factors you must consider in selecting the proper axle gears for your truck, including:
There are 3 major gear sets in your 4x4 that define its performance: 1) the transmission, 2) the transfer case, and 3) the gears inside your axles (which are housed within the differentials). One popular measure of your rig's trail capability is its crawl ratio. A crawl ratio is the lowest combined reduction you can get from your rig by putting it into first gear, low range. Crawl ratio is calculated by multiplying your transmission's first gear, times your transfer case's low range, times your axles' gear ratio. For example: a stock TJ Sport with an AX-15 manual tranny (3.83-to-1 first gear), an NP231 (2.72 low range), and 3.07 gears in the diffs has a crawl ratio of 32-to-1 (3.83 times 2.72 times 3.07 = 31.97). The higher the numerical representation of your crawl ratio, the slower your Jeep will go. For example, a Jeep with a crawl ratio of 100:1 will go much slower than one with a the 32:1 ratio. So what is the best crawl ratio to have? Well, it depends. Here are a couple of rules of thumb:
Differential gears are the most commonly swapped of the three major gear sets. Mainly because they offer the greatest number of options for fine tuning your Jeeps final-drive ratio and crawl ratio. They are also the least expensive, especially if you have the tools and mechanical ability to install and "set them up" yourself (not an easy job, IMO). Expect to pay an average of $250 per gear set (you will need a set for each axle), plus another $250 per axle (average) for professional installation, for a total cost of around $1,000. That doesn’t sound very inexpensive to me, you say. I didn’t say inexpensive; I said least expensive. Here are your other re-gearing options. You could modify your factory NP231 transfer case with a TeraLow 4:1. That will run you an average of around $1,500, including professional installation. However, it will only be effective when your transfer case is in low gear. That means your Jeep (with its factory 3.07s and those 35-inch Swampers you recently installed) will still a PIG on the highway! The only other viable alternatives would be to completely swap out the transfer case and/or the transmission. Now you’re talking BIG bucks, not to mention various other modifications you would have to make in order for these swaps to work properly. Okay, back to our discussion of differential gears. Differential gears consist of a ring gear (the larger gear) and a pinion gear (the smaller gear), which operate perpendicular to each other. The drive shaft rotates the pinion gear in the same plane as the engine and transmission, which in turn drives the ring gear (which rotated in the same plane as the axles and wheels). Remember, the lower the gear ratio, the faster the smaller gear has to rotate in order for the larger gear to make one revolution. Since ring gear diameters are normally constant (for any given axle), the lower the gear ratio, the smaller the pinion gear diameter is. It follows that the smaller the pinion gear, the fewer teeth it has, which in turn creates more force on each tooth. That's why you often hear that the lower the gear ratio, the weaker the gears. Well, they aren't actually any weaker, it's just that there are less teeth across which to spread the force. Hence, you are more likely to chip or break pinion gear teeth on lower gear sets. So how do you tell what gear ratio you currently have in your axles? Often, there will be a little metal plate attached to the diff cover bolts that will identify the gear ratio. However, if you purchased a pre-owned Jeep, the previous owner may have re-geared it. Here's a simple formula that will get you pretty close: Gear Ratio = (rpm * actual tire diameter) / (mph * 336) For example, my 33" x 12.50" BFG MTs are actually only 32 inches tall. On the interstate, I set my engine speed to 3000 RPMs, which gave me an indicated speed of 63 MPH in 4th gear (a 1:1 gear ratio). Plugging the numbers into the formula: Gear Ratio = (3,000 * 32) / (63 * 336) Since they don't make 4.61 gears, I checked the top row of the Gear Ratio/Tire Diameter Table below for the gear size that most closely matched my answer; which is 4.56. And that's exactly what I have in my diffs! Remember, this formula assumes a 1:1 tranny ratio (i.e., make sure you are not in overdrive, or 5th gear in a manual tranny), actual tire diameter (not what's printed on the side of the tire); and an accurate tach and speedometer. Gear Ratio/Tire Diameter Table This table can be used as a guide to estimate your engine speed (in RPMs), with various gear ratio and tire size combinations. The data was derived using the Gear Ratio formula above, solving for the "RPM" variable (i.e., rpm = ((mph * 336) * gear ratio) / tire diameter). These calculations assume a transmission ratio of 1:1 (e.g., you are not in overdrive or 5th gear), actual tire diameter (vs. what's printed on the sidewall), an accurate tach & speedometer, and traveling at approximately 65 mph. The colors represent various power bands, depending on your needs and desires. The yellow group will result in less power, but should result in better gas mileage on the highway, at around 2600 RPMs. The green group will get you relatively close to the factory specs of 2800-2900 RPMs. Finally, the red group will give you more power, albeit at the expense of gas mileage. To use the table, find your tire diameter in the far left column. Then scan to the right to find the RPMs you would like to attain based on your desire for better gas mileage, factory specs, or more power. Finally, go straight up that column to find the appropriate gear ratio to install in your front and rear differentials.
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Copyright 1998 - 2008 Ó by Glenn Bontly