Introduction
The Quest For SLOW
For years now, the hot ticket for better, more controlled rockcrawling has been low gears. Lower speeds translate to greater driver control and less likelihood of breaking traction. Traditionally, the only routes to lower gears have been through the transmission and differentials. Due to the technical and financial hurdles involved in swapping transmissions, the most common choice has been to change the ring and pinion gears in the differentials.
The main drawback to this approach, however, is that the lower gear ratio affects the offroad AND onroad performance. Low gears are fun when you can idle up rock faces and a major annoyance when your engine is buzzing at 3000 rpm while you're trying to keep up with traffic on the freeway. For most of us who drive our vehicles to work during the week and offroad during the weekend, ring and pinion re-gearing isn't the ideal solution. Generally speaking, they're good for restoring your vehicle's performance after you've installed taller, heavier tires. But for really, really low gearing, something else is needed.
Fortunately, Tera Manufacturing has the answer. They manufacture super low gear sets that swap into the NP231, Dana 20 and Dana 300 transfer cases.
In this article, we cover the installation and trail testing of the TeraLow Low300 gear kit for the Dana 300 in Project YJ.
Final Gearing
The Project YJ Jeep has a TorqueFlite 999 automatic transmission with a 2.74 first gear. The torque converter provides another 2:1 of reduction but only when driving the transmission. Under compression braking, the torque converter effectively HALVES the reduction. The net result was that I spent a lot of time riding the brakes when going downhill or crawling over rocks. The Jeep was too fast even in low range.
The rule of thumb for serious rock crawling is that a good final gear ratio is 45:1 or higher (numerically). Project YJ has a ratio of 29.4:1.
The ratio is calculated by multiplying the first gear, transfer case low range and differential gear ratios together:
2.74 x 2.62 x 4.10 = 29.4
With the addition of the Tera 4:1 gear set, Project YJ would get a final ratio of 44.9:1.
2.74 x 4.00 x 4.10 = 44.9
This is a 52.7% improvement. With the torque converter factored into the equation, we would realize a driving reduction ratio of approximately 90:1 and an engine braking reduction ratio of 22:1.
Finally, it would be in the same gearing territory as the serious rock crawling vehicles.
Gearing Calculator
You can calculate your own final gear ratio by using this calculator. Type in the values and press CALCULATE.
