A Lesson On Gearing
(Not meant to be a end all explanation, just enough to get you over the rough spots…)
By Doug Chartier

**This is a companion article to one printed previously in 4x4 Low Down and available in the SHR website Tech Section -  Understanding Gear Ratio’s vs. RPM http://www.southernhighrollers.com/tech/articles/gearratios.html  

All the gears, including 1st, 2nd, 3rd, 4th, and 5th (or even D, R, etc on an Automatic) will be lower when using the 4:1 kit and lowering your low side in the transfer case.  The transfer case and the transmission are two different pieces of machinery.  The transmission takes rotation of the engine crankshaft as input.  You change that rotational speed with your throttle.  The rotational speed comes in, and the transmission alters it based on the gearing.  Suppose 1000 rpms come into the transmission.  If your gear was a 2:1 ratio, that means that it reduces the input by that ratio - for each 2 revolutions that come in, one revolution is passed to the output shaft, 1000 rpms out of the engine and into the transmission, 500 rpms out of the transmission.  A gear of 4:1 would take the same 1000 rpms out of the engine and output 250 rpms from the transmission. 

Over the years different gears have been given specific names.  "Direct" or "high" gear is normally associated with a 1:1 ratio (1000 rpms in - 1000 rpms out), "overdrive" with any gear less than 1:1.  A .79:1 (1000 rpms in - 1255.82 rpms out) would be an overdrive gear.  On the low side any gear that is lower than one normally used for starting from a dead stop is called a "granny" gear, "low low", "compound low or just compound"; although “compound” is often associated with another low gear lower than the "granny".  Heavy equipment, trucks etc., may have several gears that are lower than the "low" gear or "first" gear. 

Ok, now we have the operation of the transmission.  On a normal 2-wheel drive vehicle the output from the transmission goes directly to the axle where the rotational speed is changed again.  Suppose the transmission is exporting 1000 rpms.  If the gear in the axle is 2:1, we have a further reduction of rotational speed - 1000 rpms into the differential, 500 rpms out to the wheels. 

Go back to the engine.  1000 rpms out of the engine into a 2:1 transmission produces 500 rpms out to the axle.  Using the 2:1 in the axle again causes the 500 input to be reduced by half to 250 rpms from 1000 down to 250 is a 4:1 ratio.  If you multiply the axle ratio by the transmission ratio, you get 4:1. 

And finally, we introduce the transfer case.  The transfer case is a 2-speed transmission.  It's just another transmission between the main box and the axle.  In addition it outputs from the front and the rear so that you can operate 2 axles from it.  It takes input from the main transmission, alters it, and transfers it to the axle or axles.  The transfer case has 2 gears, low and high, and again the "high" conforms to the above mentioned specific gear name.  High gear is 1:1 or direct.  It does not alter the input, so it is as if the transmission was connected "directly" (direct) to the axles.  If you are in high gear in the main transmission, it is as if the engine is connected "directly" to the axle because you could accomplish the same thing with no transmission at all - a direct connection. 

The final bit of math.  We have 1000 rpms coming out of the engine.  The transmission is in a 2:1 gear.  500 rpms out of the transmission and into the transfer case.  The transfer case is in low gear, and for this explanation, it will be another 2:1 ratio.  So, 500 in, 250 out.  The axle is also 2:1.  250 in 125 out.  1000 rpms out of the engine and 125 rpms to the wheels.  That is an 8:1 ratio.  If you multiply all the gears - 2 X 2 X 2 you get 8.  If you reduce the 1000:125 ratio to it's lowest form, you get 8:1.

A real life example: I have a 4.3:1 ratio on the low side of my t/c and 4.56:1 gears in the axles.  The low gear ratio in the transmission is 3.83:1.  If you multiply that out, you find that the overall ratio, the "crawl" ratio in low gear, is 75.09:1.  For each 1000 rpms of the engine the wheels turn 13.31 rpms.  A stock Jeep with a 3.73:1 axle ratio would have a 37.57:1 crawl ratio - 2.63 (transfer case) X 3.73 (axle) X 3.83 (low gear in the transmission).  In overdrive and the low transfer case gear my Jeep will only run about 18 mph.  I often have to change into the high side of the transfer case on a flat trail.

 Back to the big trucks for a moment.  Where the Jeep has a 2 speed transfer case or second transmission, big trucks have a 5 or 6-speed second transmission, and sometimes even 3 transmissions as well as selectable gears in the axles.  (Note: Sometimes on off-road vehicles especially it seems on Toyotas you will see a setup with multiple transfer cases sometimes referred to as “Crawl Boxes”.  These adapters and cases come from many sources but Marlin and Klune are two of the better known ones.

I might add that the term "crawl ratio" is one invented by off roaders or 4 wheelers.  It has no official or semi official meaning that I know of.  More correctly the term should be "final ratio" or "overall ratio" because it refers to the cumulative ratio of the various gears from the power source to the point where the work occurs - engine to wheels - regardless of which gear is used. 

This ratio is useful to 4 wheelers to determine the lowest overall gearing of the vehicle, but it is just as useful to answer your original question.  You questioned the drivability of your vehicle in the higher gears on a relatively smooth trail where low speed was not a factor. 

Let's do a bit more math and combine the results with real world data.  The following are not real numbers, but they will make the point. 

The 4th gear of a standard shift 5 speed Jeep transmission is 1:1 or "direct" or "high gear".  That is a real number, BTW.  Another real number is the 2.63:1 gear ratio in the low side of your transfer case.  A Jeep TJ Transfer Case has a ratio of 2.73:1.  Suppose you are able to drive comfortably at 30 mph in 4th gear/low side transfer case.  Your concern was whether you could continue to drive on the smooth trail in low range, or whether you would have to change into high range if you changed to a 4:1 gear in the transfer case. 

Here's the math.  The 30 mph is a number from thin air, but will make the point.  The overall or final gear ratio in your current vehicle using 4th gear, the low side of the transfer case and we will assume a 3.73:1 axle ratio (that's a common ratio.  I don't know what yours is.) is 1 (transmission gear) X 2.63 (transfer case) X 3.73 (rear axle) or 9.8099:1.  So, at a particular rpm you can drive 30 mph on the trail.  The question is - at the same rpm how fast would you be going with a 4:1 low in the transfer case instead of a 2.63:1. 

The easy way to figure that is - a ratio of ratios, or 4:1 is to 2.63:1 as "X" is to 30, or 2.63 is to 30 as 4 is to "X".   The math:  2.63 divided by 4 is .6575 multiplied by 30 is 19.725.  If you changed from 2.63:1 to 4:1 in the transfer case, and you formally went 30 mph in 4th gear, you can now go 19.725 mph in 4th gear at the same rpms.  If you changed to a 4.3:1 ratio, your speed at the same rpms would drop to 18.35 mph.  OR you could do it using the "overall ratio".  The final or overall ratio of a 4:1 under the same circumstances would be 4 (transfer case) X 1 (transmission) X 3.73 (axles) or 14.92:1.  Again we look at the relationship of the ratios.  9.8099:1 is to 30 as 14.92:1 is to "X".  The math: 9.8099 divided by 14.92 times 30 equals 19.725 - same result. 

Having the real world data of your own Jeep - all the transmission ratios, the axle ratios, the transfer case ratios and the speeds at various rpms in various gears will allow you to mathematically determine the speeds under identical conditions if you only change one factor - a single gear ratio - the transfer case gear ratio, for example.  You can do it very quickly as you drive if you use a calculator.  Predetermine the relationship of the two transfer case ratios.  If it is 2.63:1 vs. 4:1 the number you will use is .6575 - 2.63 divided by 4.  If the ratio is 2.73:1, the number would be .6825.  As you drive attain a comfortable speed in each gear.  That would be a speed that does not over-rev the engine.  If it is comfortable to drive 27 mph in 4th, simply multiply 27 by .6575 or whatever the correct number is.  The answer will be the speed at which you will be comfortable in the same transmission gear with a lower gear in the transfer case.   

How to figure your vehicle crawl ratio:  

__________  x  __________  x  __________  =  __________
1st Gear             Low Range             Axle                  Crawl
Transmission       Transfer                Ratio                 Ratio
                              Case                              

Of course the one thing overlooked on this overall ratio is tire size.  Tire size plays a lot into the off-road performance of a particular crawl ratio.

 For example if I had a Jeep TJ with a stock 5-Speed, a stock t-case. 4.10 axle gears and 33” tires. I would have a crawl ratio of 43.98849. 

In order to “upgrade” to 35” tires and keep the same performance I would need a “crawl ratio” closer to 46.6545.  

I can determine this by a the following formula: 

(__________  x  __________ ) /  __________  =  New
Current                 New Tire             Old Tire                Crawl
Crawl Ratio          Size                     Size                     Ratio

(43.98849x35)/ 33 = ~46.6545 

In order to achieve that crawl ratio one could either modify the transfer case to a numerically lower gear or the axles. 

One more bit of info.  The lower gears multiply the power delivered to the wheels.  In the 4 wheeling community the goal is to drive VERY slowly.  High gears will only allow a vehicle to drive at a certain low speed before the engine stalls.  Lower gears allow the vehicle to drive at a lower speed before stalling the engine.  Our goal is to drive SLOW.  Trucks do the same thing.  They gear LOW, but their goal is not slow speed.  Their goal is to increase the power to the wheels.  Different goals, but the results of each satisfy the goal of the other.  In some respects they are counter productive though.  The 4 wheeler starts breaking things because of all the extra power that results from his "go slow" low gears, and the trucker has to shift a whole bunch of gears to get up to speed because his quest for power has resulted in a very low maximum speed in the low gears.  Some of the very low gears in a big trucks only have a speed range of a very few miles per hour - the lowest, from 0 to about 2 or 3 mph, the next from 0 to about 5 mph etc.  When you get into the higher gears they are only good for ranges of speed like from 34 mph to 45 mph, but that's a factor of the engine power band.  The engines, despite their size and power, cannot "pull the load" if the engine revolutions are allowed to go too low.  Same is true of your Jeep.  Overdrive will not "pull the load" below a certain speed.  The engine is not strong enough.