4WD Glossary

larger tires can be good

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4WD seminar

To get a number for the "crawl ratio" or "overall gear ratio" of the low range setting in 4WD, first gear of transmission, transfer case ratio (in 4Lo) and axle differential ratios are multiplied. (For vehicles with automatic and/or geared hubs those ratios need to be factored in as well).

A good 4WD with manual transmission should have a crawl ratio of 40:1 or better, with an automatic transmission 30:1 or better.

 

Every gear ratio involved subsequently reduces the rpm from engine to wheels. With the engine at 1000 rpm and a gear ratio of 42.8:1 each tires rotates only 23 times per minute. So, the lower the crawl ratio is (numerically higher) the slower a vehicle can "crawl" in first gear and low range.

 

Every gear ratio involved subsequently boosts the torque output. This engine produces 200 lbs./ft. of torque - transmission and transfer case multiply it to 2084 lbs./ft. and the differentials boost it further so that each wheel can create a whopping 2141 lbs./ft. of torque. So, the lower a crawl ratio, the higher the torque available at the wheels.

For extreme off-road driving an even lower crawl ratio is needed

The "pumpkins" in the picture above are the axle differentials.

Please note: Mounting larger tires will have a negative effect on your crawl ratio.