Alex, the tractive effort at the contact patch is a function of all the ratios between the flaywheel and the wheel, as well as the wheel (tire and rim) radius. Effectively, you multiply all the ratios together, multiply that by the engine's torque (at the specifik revs you are interested in), and then divide that by the wheel radius to calculate how much force is available to push the car forward.
As an example, my car has a first gear ratio of 4.556:1, a low ratio of 2:1 and a diff ratio of 4.111:1. Thus the total gear ratio in 1st Low is 4.556x2x4.111=37.46:1 Now let's say I have 350N/m of torque @1500rpm and my tires are exactly 33" in diameter, I would thus have 37.46x350=13110Nm at the wheels and 13110/0.491=31283N available at the contact patches. That is 3128kg of tractive effort. In other words, assuming I have enough traction, I could just-just climb a wall at 1500rpm. But ideally I want to be crawling at 600rpm, and there I have less than half that torque, and you probably have even less. Speed can be calculated in the same manner, and is perhaps more important here, as the slower you go the more control you have.
The crux, however, is this: if you are happy with the on-road performance of your car with your chosen tires, then there would be no need to change the diff ratios. A change in transfer case ratios would thus be simpler and cheaper.
Please note that what we are discussing here is a change in transfer case ratios, not a crawler gear. A crawler gear is actually another 2-speed gearbox fitted between the normal gearbox and the transfer case. It gives you another whole set of ratios to play with, and a third gear lever.
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