Lets begin with looking at the big picture.
Just like a car a 4x4 has an engine and a transmission. In addition it has a transfer case moving pover front and rear.
Taking my own 4x4 as an example it has a transmission with the lowest gear of 2.47 and the 4Lo is 2.48. My differentials are 4.88. Multiplying these three ratios gives me the overall gear ratio or crawl ratio of 29.77 - a reasonably low number for a 4x4 with an automatic transmission.
When getting bigger tires for your 4x4 it is easiest to change the gear ratio in the differential.
When you plan on getting bigger tires you have to calculate a new gear ratio for your differentials. This is both for performance as well as fuel economy.
You begin with multipling your new tire size with your old gear ratio and dividing the result with the old tire size. This is a simple formula that gives a pretty accurate result. Continuing with the example I multiply 38 with 3.91 and divide the result with 30 giving me the number 4.95 where the closest awailable gear ratio is 4.88.
The gear ratio most interesting to 4x4 truck owners is the ratio between the driveshaft and axels. It is controlled by the number of teeth in the ring vs. the pinion.
Lets look at these two perfectly rount gears. The smaller one is the driving gear and the larger the driven gear. If the smaller one has 10 teeth and the bigger one 20 the gear ratio between them are 20 to 10 or 2 to 1.
The differential works in the same way except the directions of the axles are at a 90° angle. So the pinion is an extension of the driveshaft and the ring is around the axles. In this case the pinion has 8 teeth and the ring 39 teeth. Dividing gives us the real ratio 4.875
A list of the 4x4 gear ratios used in popular transfer cases and transmissions.
When you lift your 4x4 truck you consider your options for lowering the overall gear ratio. In addition to the ring and pinion gear ratio in the axles you may consider other options for your transfer case and transmission.
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|Dana 300 - std.||2.62|
|Dana 300 - 4:1||4.00|
|Borg Warner 1356||2.69|
|Toyota (67-74 3 speed)||2.31|
|NP 231 - std.||2.72|
|NP 231 - 4:1||2.72|
|Muncie SM 420||7.00||3.60||1.70||1.00||7.05|
|Muncie SM 465||6.55||3.58||1.57||1.00||6.09|
|New Venture NV3500||4.02||2.32||1.40||1.00||.73||5.04|
|New Venture NV4500||5.61||3.04||1.67||1.00||.73||5.63|
|GM 4L60 / 4L60-E||3.06||1.62||1.00||0.70|
|GM 4L80E / 4L80-E||2.48||1.48||1.00||0.75|
|Ford C4, C5 & C6||2.46||1.46||1.00|
|Chrysler Torqueflite 727||2.45||1.45||1.00|
Choosing the best gear ratios for your 4x4 truck - The correct chart for your gear ratio is important for drivability and economy. When you increase the tire size without changing the gear ratio you increase fuel consumption and you can not drive at the low speeds needed for the off roads.
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