Gear Ratio Chart - Choosing the best gear ratios for your 4x4 truck

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. A ratio of 4.56 means that as the axle turns one whole circle the driveshaft turns 4.56 circles. The higher the number the lower the ratio becomes.

Chart Gear Ratio

The correct 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.

 

A simple formula

The most simple formula for the correct gear ratio is this:

New Tire Size x Old Gear Ratio
New Gear Ratio =――――――――――――――――
Old Tire Size

Example:

The original tire size is 30" and the original gear ratio is 3.91. You plan on moving up to 38". Then the formula would be calculated as: (38*3.91)/30 giving you 4.95. Not all gear ratios are available so you have the choice between 4.88 and 5.13, 4.88 being the closest.

Choose a little bit lower gear ratios if you do a lot of off-road driving and a little bit higher if you do more long distance highway trips.

For calculating this formula on-line go to the Tire Size Calculator.

An old gear ratio formula

To calculate the rotations pr. minute of the engine you can use this formula.

This formula is for 4x4's with a 1:1 final drive ratio (no overdrive). It should work fine for manual transmission as well as an automatic when they are not in overdrive.

MPH x Gear Ratio x 336
RPM =――――――――――――
Tire Size

Example:

Adding to the last example. You usually start with 65 MPH, since that is the natural highway speed, multiply by 3.91 and then again with 336, dividing all by 30 gives you the before RPM of 2846. Calculating for the after ratio of 4.88 gears and 38" tires gives the RPM of 2805.

A Gear Ratio Table

To sum it all up it can be handy to use this table. It is based on the above formula. You can use it to quickly estimate your trucks best gear ratio.

Gear Ratio
Tire
Size3,313,423,553,733,914,114,274,564,885,135,295,385,716,177,17
30241024902584271528462992310933203553373538513917415744925220
31233224092501262827552896300832133438361437273790402343475051
32225923342423254626692805291431123331350136103672389742114894
33219122632349246925882720282630183230339535013561377940834745
34212621972280239625122640274329293135329533983456366839634606
35206521342215232824402565266428453045320133013357356338504474
36200820752154226323722493259027662961311232093264346437434350
37195420192095220223082426252026922881302831233176337036424232
38190219662040214422472362245426212805294830403092328235464121
39185419151988208921902302239125542733287329623013319834554015
40180718671938203721352244233124902664280128882937311833693915
41176318221891198720832189227524292599273328182866304232873819
42172117781846194020332137222023712538266827512798296932083728
43168117371803189419862087216923162479260626872733290031343642
44164316981762185119412040211922632422254626262670283430633559
45160616601723181018981995207222132368249025672611277129953480
46157216241685177118561951202721652317243625122554271129293404
47153815891650173318171910198421192268238424582500265328673332
48150615561615169717791870194320752220233424072448259828073262
49147515241582166317431832190320322175228723582398254527503196
50144614941551162917081795186519922132224123112350249426953132
51141714651520159716741760182919532090219722652304244526423070
52139014361491156716421726179319152050215522222260239825913011
53136414091463153716111694176018792011211421802217235325432955
54133913831436150915811662172718441974207521402176230924952900

 

Choose lower numbers for better economy and highway usage and higher numbers for towing and better off road capabilities.

Thrandur Arnthorsson
Thrandur is the chief-editor and owner of 4x4 Off Roads and an 4x4 off road enthusiast. Living in Iceland he loves to share the coolest 4x4 trucks and supporting more offroad freedom. Thrandur is the author of the Udemy course How to modify your cool 4x4 truck!.

For more calculations go to the Tire Size Calculator.

 

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