Every component of a 4x4 is in some way responsible for the vehicle's operation. You can't spin wheels if you don't have axles, can't spin axles if no transmission to transmit torque, and can't transmit torque if you don't have an engine to create it in the first place. Basically, many, many factors are responsible for every action that occurs. The same can be said about calculating your vehicle's crawl ratio.
|CRAWL RATIO FORMULA|
|trans ratio x transfer case ratio x diff ratio = crawl ratio |
|A x B x C = D|
|A = 4.04 (NV3550)|
|B = 2.72 (NP231)|
|C = 4.56|
|Solve for D:|
|4.04 x 2.72 x 4.56 = D|
|D = 50.1|
The crawl ratio for this vehicle setup is 50.1:1. This is a fairly reasonable figure, but a lower crawl speed (numerically higher number) will effectively create a more capable 4x4 for low-speed wheeling. Installing 4:1 gears in the transfer case will create a 73.7:1 crawl ratio. Install a lower-geared transmission such as the T-18 with a 6.32:1 First gear ratio along with the 4:1 T-case gears, and the crawl ratio improves to a whopping 115.3:1 crawl ratio. With the addition of a lower-geared transmission, however, use of higher gearing in the axles is made possible. For this reason, the 4.56 gears would likely be replaced with 4.11 gears and would result in a 103.9:1 crawl ratio. The 100:1 range is about what most strive for in a rockcrawling rig. You can certainly get away with a much higher (numerically lower) crawl ratio, but the lower you go, the greater the odds of not having to back out of any obstacles on the trail.
Crawl speed is the measure of time it takes your vehicle to move a set distance. It is expressed as a numerical figure in feet per minute (fpm). It can be determined mathematically by calculating a variety of factors, including tire diameter, engine rpm at idle, axle ratio, and Low transfer case gear ratio. Tire diameter is not a constant value since the true diameter changes as you air down for the trail or as the vehicle load increases and decreases. You can use either the tire manufacturer's indicated size (some publish the actual tire size figures) or measure it for yourself.
Crawl Speed Formula
A = tire revolutions per minute
B = inches per tire rotation
C = feet per minute traveled
Solve for A:
rpm · First gear trans ratio · T-case ratio · axle ratio = A
Solve for B:
tire circumference = diameter x 3.14159 = B
Note: 3.14159 is the numerical value of pi. Pi is the ratio of the circumference to the diameter of a circle, which is approximately equal to 3.14159265358979323846 to an infinite decimal. Pi is always the same number, no matter what circle you use to compute it.
Solve for C:
(A x B) · 12 = C
First gear trans ratio: 4.04 (NV3550)
T-case ratio: 2.72 (NP231)
Axle ratio: 4.56
Tire diameter: 35 inches
Solve for A:
1,000 · 4.04 · 2.72 · 4.56 = A
A = 19.96
Solve for B:
35 x 3.14159 = B
B = 109.96
Solve for C:
(19.96 x 109.96) · 12 = C
C = 182.9
Crawl speed with this vehicle set up at 1,000 rpm is 182.9 fpm or 3.05 feet per second (fps). The addition of 37-inch tires would alter this figure to 193.2 fpm (3.22 fps). The addition of 4:1 transfer case gears lowers the figure to 124.2 feet per minute or 2.07 feet per second. Changing axle gears or transmissions will also alter this figure.
A vehicle's crawl ratio represents the lowest gear ratio that can be achieved. Installing lower gears in the transfer case isn't the only way to alter your crawl ratio. You can change the axle ratio, adjust tire size, or even swap in a lower-geared transmission. An ideal crawl ratio for general-use 4x4s is typically in the 50:1 range. Those who tackle more difficult terrain will strive to obtain at least a 100:1 ratio.
Crawl speed is another bit of terminology that can be helpful to know. This figure represents the amount of time it takes your vehicle to travel a particular distance, in this case measured in feet per minute (fpm).
Use the accompanying formulas to calculate your vehicle's crawl ratio and speed. If you're not confident about your math skills, then you can try one of the available online calculators that do the math for you. Novak Conversions (www.novak-adapt.com) offers one such calculator as does Marlin Crawler (www.marlincrawler.com).
Popular transfer case low ratios
Dana 300: 2.62
Dana 20: 2.46, 2.34
Dana 18: 2.46
Land Cruiser: 2.31 ('64-'73), 1.99 ('73-'75), 1.96 ('75-'80), 2.28 ('80-'87)
Toyota p/u & 4Runner: 2.28 ('79-'95), 2.57 ('87 and later)
Popular Transmission First Gear Ratios
AX-4/AX-5: 3.93, 3.83
Ford T-18: 6.32
Jeep T-18: 4.03
T-19: 4.00, 5.11
T-90: 2.98, 3.44
Toyota four-speed: 3.67 ('79-'80), 3.93 ('81-'82)
Toyota five-speed: 3.93 ('81-'95), 3.95 ('85-'95 EFI), 3.83 ('88-and-later R151F), 4.31 ('86-'87 turbo)
Toyota Tacoma five-speed: 3.95
Land Cruiser: 2.75:1 (3SP), 3.55, 4.93 (4SP)
Ford AOD: 2.40