Since crawl ratio of a vehicle represents the total reduction of the engine speed until the road wheels, it is determined by combining the contributions of different elements on the entire drive train, including transmission and differential, both of which typically introduce a certain amount of speed reduction.
Since a lower crawl ratio (higher gear ratio) implies a larger output torque on the road wheels, it is desirable for vehicles that need to pull large loads, climb steep inclines, or drive over obstacles on the road or terrain, such as rocks, which is sometimes referred to as crawling over the rocks.
Therefore, crawl ratios are most often discussed for large SUVs, trucks and off-road vehicles.
Note that tire size (or dimensions of the road wheels) does not affect the gear ratio of a vehicle, and thus using a different size tire on the same vehicle does not affect the torque on the road wheels or the crawl ratio.
However, for a given engine speed and a gear ratio, the output force on the road wheels decreases as the tire size increases.