Off-Road Gearing – Simplified
What affects the gearing of a 4WD?
Gearing is all about getting the power from the engine to the ground. The simplest example is your old 10 speed bicycle. In the lowest gear, you could climb hills while making the most of your effort. On flat land, however, this lower gear would waste your effort because the higher gear can go further with each push of the pedal.
A 4WD vehicle is more complex, with the power from the engine transferring through several different parts. The components that will affect your 4WD’s gearing are: the transmission, transfer case, differentials, and tire diameter. You can greatly improve your vehicle’s performance by understanding how all these parts work together.
What should I consider first?
With gearing, there are no absolutes, no right or wrong answers. You can gear your 4WD so that it crawls over the greatest obstacles with ease but is so bad on the interstate you have to haul it. Or, you can gear your 4WD so that it gets amazing gas mileage and flies down the interstate, but it can barely hop a curb. Most people would want their vehicle geared somewhere in-between these two extremes. That said, the first thing you should consider is how you want to use your 4WD. You can then play with the numbers until you find your sweet spot.
What is a crawl ratio?
The 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 your 4WD will perform under different circumstances. The higher the crawl ratio, the more control you’ll have to go slowly (crawl) over obstacles. A lower crawl ratio will be great for daily driving. Like most things, you can have too much of a good thing. A good scale to go by is:
- Under 50 Crawl Ratio - 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
With a 4WD in the lowest possible combined gear, you would compare the torque at the wheels with the torque at the flywheel to get the crawl ratio. You can use the formula below or an online calculator (https://www.crawlpedia.com/crawl_ratio_calculator.htm) to calculate the crawl ratio.
Let’s compare a Jeep Wrangler with a factory AX15 5-speed transmission and factory transfer case and differentials to a TR-4050 transmission with the same transfer case and differentials.
Crawl Ratio = 3.83 X 2.72 X 3.73 = 39
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%. We can adjust the crawl ratio even more by changing the gears in the transfer case with a kit from Advance Adapters (https://www.advanceadapters.com) and swapping out the gears in the differentials. Let’s see where that would take us:
TR-4050, New transfer case gear, New differential gears
Crawl Ratio = 6.16 X 3.15 X 4.11 = 80
As noted, 80 is ideal for intermediate trail use, so 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 obsticals on an intermediate trail ride.
What about tires?
Tires may not effect the Crawl Ratio, but they still have an effect on the performance of your off-road vehicle. Larger tires can provide clearance and traction, but they will also give your 4WD sluggish acceleration. The larger the tire, the more force required to turn the axle. For the axle shaft to turn the larger tire, it will need to be geared lower so more torque is transferred to the wheels.
In axles, the lowest gears have higher numbers. For example, if you have 3.73 rear axle ratio and 33” tires, you’re going to have very poor acceleration. To correct it, you could swap out to a lower 4.27 rear axle gear and get more torque and faster acceleration.
While the transmission and transfer case may have gotten your crawl ratio to a good number by themselves, if your tall tires make you sluggish, you’ll want to go deeper in the rear axle ratio.
You can use this calculator to see how your rear axle ratio and tire size will affect your RPMS. https://tiresize.com/gear-ratio-calculator/
Don’t stall the engine!
Crawling over extreme obstacles requires control and very slow speeds. You need to let the RPMs of the engine remain high enough to keep the engine running (usually at least 6-800 RPMs) but use gearing to slow the wheels down and provide additional torque. The higher the crawl ratio, the lower the rotational wheel speed.
However! Large tires can speed up the tire RPM. You’ll need to increase the crawl ratio to compensate for the larger tires. You can calculate the minimum wheel speed with these formulas:
Minimum Wheel RPM = Minimum Idle RPM / Crawl Ratio
If we have a 33” tire, 800 idle speed, and a 80 Crawl Ratio, this is what our max speed would be:
800 RPM/80 Crawl Ratio = 10 Minimum Wheel rotations per minute
Find the circumference of the tire (Circumference = Diameter times Pi)
33” diameter tires times pi (3.142) = 103.69”
Multiply the circumference by the minimum wheel RPM to find the surface speed.
10 x 103.69” = 1037 inches per minute/12 = 86 feet per minute
Divide by 5280’ which is how many feet are in a mile.
86/5280 = We can crawl as slowly as .02 miles per minute, or approx. 1 mph .
Calculate your top speed
33” tires, a TR-4050, 3.15 transfer case, 4.11 rear axle ratio gave us a great crawling speed, but what is our top speed? We don’t want to top out at 40 mph and listen to the engine whine on the interstate.
You can use our speed analyzer to calculate your top speed: https://shiftsst.com/speed-analyzer
With the engine at 2250 RPMs, 33” tires, 4.11 rear gear, and the TR-4050’s .76 5th gear, we can cruise at 70 mph. Not bad!