Gearing you 4x4 II


Well we have gone through the basics of gearing and tire sizes and the affect they have
on our rigs, but that is the on road part of our vehicles.
Time to get into the off road part, since you didn’t just mod your truck with great suspension, 
tires, gears and traction devices to just drive it on the road???


Transfer cases
What is a transfer case and what does it do?
In short it is nothing more than a system to split the drive between the front and rear axle. There are many variations, drive styles are either chain or gear drive, passenger or driver side front drive outputs. Almost all cases have a high and low range, there are some that are AWD (all wheel drive) with no range selection but these are not suited to mods or off roading so they will not be mentioned anymore than this. As you figure high range on most cases is 1:1, there are exceptions (like Suzuki Samurai’s) but more on that later. It’s low range and the effect it has on the trucks performance makes the biggest difference.When you engage low range power is transferred through another set of gears in the transfer case that lower your speed but increase the available power to the wheels.

Low range ratios
Different transfer cases have different ratios, it depends on the make and model or if the internal gears have been changed.
For an example I am going to use a Jeep as they are probably the most common modified and or associated with off road then there are 2 imports that have a one up on the jeeps.

If you look under a Jeep YJ, TJ you will likely find a NP231. These are a chain drive passenger side drop transfer case with a low range of 2.72:1. If you remember from our gearing part one lowering your gears means slower speeds. With a 231 case, an AX15 transmission and 4.10 gears you get a crawl ratio of 42.7:1 (note: If you have an auto trans it is harder to calculate as the first gear is higher on the transmission but the multiplying factor of the torque converter makes up for that.). Crawl ratio??? What the heck is that???  This is the lowest ratio you have with your rig in 1st gear, and the transfer case in low range. In numbers it will look like this: Trans 1st gear x t-case low range x axle ratio 3.83 x 2.72 x 4.10 = 42.7:1 Ok, great so what does that mean? The lower you can get your crawl ratio the better your rig will crawl over an obstacle. If you think of a speed bump the slower you drive over it the less harsh it will be to the truck and it’s occupants. Not all transfer cases are created equal though, with the introduction of the Jeep Rubicon and the RocTrac transfer case with it’s 4:1 gear set has changed things for Jeep owners, considerably. If you were to compare the same numbers above with a Rubicon’s t-case the new ratio will be 62.8:1. To compare the two if you had two Jeeps wearing 31” tires (remember measured 31”s) The RPM differences between the two driving at 3mph are:
With 42.7:1   you will be running 1389 rpm
With 62.8:1   you will be running 2042 rpm
That is almost a 700 rpm difference. With the 62.8:1 the same engine speed will net you a ground speed of 2 mph. That is about 75 feet per minute slower. You can lower your axle ratios and get the same effect but if you remember part one there is only so far you can go before you sacrifice your on road driving ability.

Aftermarket gears
There are ways to lower you crawl ratios, some can get very expensive very fast. There are different low range ratios available for select cases or there are doublers available for some applications. For Jeep owners about the best you can hope for is 4:1 unless you have the $$ for an Atlas or StaK aftermarket t-case.  For Toyota owners there are different ratios available from aftermarket manufacturers the most common is a 4.70:1. The Suzuki Samurai is the noted exception here. There are many gear sets available for these from a 4.16 up to an 8:1 low range. The exception here is that when the low range is changed the high range is changed as well due to the gear drive layout of the case. A stock case has a 1.409:1 high range and a 2.268:1 low range. If you change to a 6.5:1 gear set the high range becomes 1.7:1 For them the cost advantages of just changing the case gears vs. the axle gears is very cost effective. 





Double transfer cases
For those with a NP205 there is a kit to put a modified NP203 in front or a over/under drive from gear vendors or Klune V.  The most famous of the doublers has to be the Marlin Crawler cases for Toyotas. There is also a version available for the Suzuki Samurai using Sidekick parts. This is basically another t-case with the front drive cut off so you have two cases and two low ranges. Now your gear selections are almost limitless. With a Toyota crawler set of cases you can end up with 300, 400 even 500:1 by mixing low gears in each case and still have a high range suitable for every day driving.

What do I need…
That would be one of the hardest questions to answer as the needs of every driver and each terrain we encounter are different. For mud you would want more wheel speed to keep the tire tread clear of mud about the same will apply to sand but you can always use a higher transmission gear as well. If rock crawling is what you like the slower you can go the more control you will have and the easier it will be on you and your rig.

Thanks to: JBZukin