Getting into off-roading there are many terms that can seem like a foreign language, but things do not have to be confusing. With the help of Silver Sport Transmissions, we set out to clear up and explain crawl ratio. If you have been around a Jeeper or two the term has surely come up.
Crawl ratio is the ratio of torque at the wheels to the engine flywheel torque. A crawl ratio gives you an idea, but not a complete picture of how a four-wheel-drive vehicle will perform under different circumstances. The higher the crawl ratio, the more control the vehicle will have to go crawl over obstacles while a lower crawl ratio would be great for daily driving.
To figure out a crawl ratio you need to know the transmission first gear ratio (TR), transfer case low gear ratio (TCR), and axle gear ratio (AGR). The formula is simple once you have those numbers; Crawl Ratio = TR x TCR x AGR or use an online calculator like the one on Crawlpedia.
What Crawl Ratio Is Good For What
Under 50: Most factory setups fall here
50-60s Crawl Ratio: Backroads and light trail use
80s Crawl Ratio: Intermediate trail use
110-130s Crawl Ratio: Ideal for crawling, it allows taking obstacles at a slow, controlled speed without riding the brakes or applying constant throttle to avoid stalling.
Over 130 Crawl Ratio: Excessive with little to no additional benefit
AX15 – Crawl Ratio = 3.83 X 2.72 X 3.73 = 39
TR-4050 – Crawl Ratio = 6.16 X 2.72 X 3.73 = 62
“You can see that the TR-4050’s low first gear will improve the example’s off-road performance by about 37-percent,” Misty McComas explained. “We can adjust the crawl ratio even more by changing the gears in the transfer case with a kit from Advance Adapters and swapping out the gears in the differentials.”
When looking at the TR-4050 transmission with a new transfer case gear and new differential gears the crawl ratio improves even more. In this example, swapping out the gears in the transfer case and differentials and installing a brand new TREMEC TR-4050 5-speed makes this Jeep Wrangler better equipped to handle the obstacles on an intermediate trail ride.
TR-4050, New transfer case gear, New differential gears – Crawl Ratio = 6.16 X 3.15 X 4.11 = 80
To be able to go crawling you do not need a high-horsepower engine as there are other ways to increase the torque at the wheels with different transfer case gears and differential gears. Given this, the vehicle’s horsepower is proportional to the final torque output.
When horsepower, speed, are sent through a gear reduction such as a transmission, transfer case, or differential the output speed is reduced, but the output torque is increased. Horsepower is defined as the maximum power that an engine can put out and torque is defined specifically as a rotating force that may or may not result in motion.
When it comes to crawling over rocks, torque is important to pay attention to. An easy way to remember the difference between horsepower and torque is, horsepower is how fast your vehicle will hit a wall while torque is how far your vehicle will pull the wall.
In the end, some math we learned in high school does pay off. It may not be what most off-roaders want to do, but these are definitely numbers that should be crunched, along with the build budget. For more information on Silver Sport Transmissions be sure to visit their website.