Gear Ratio Chart: Step-by-step

Simple guide to choosing the best diff gear ratios for your Jeep or 4×4 truck

I often get questions like:

• What is the best gear ratio for 37 inch tires?
• What gear ratio do I need for 35 inch tires?
• What does a 1:1 gear ratio mean?
• What diff gear ratio is best for fuel economy?
• What is the best rear end gear ratio for my Chevy Silverado?
• What axle ratio gears should I run with for 38 inch tires on my 4×4 Ford F-150?

If you are planning on 35″ or 37″ or even bigger tires for your Jeep or 4×4 truck this guide will show you the best ratio to choose. In this gear ratio guide you’ll find the formulas and a simple chart that shows how you should lower your ring and pinion gear ratio.

1:1 Gear Ratio

The gear ratio of 1:1 means that the driven gear rotates at the same speed as the driving gear and is usually true for the ratio between the engine and the transmission when it is in 4th gear in a manual transmission. Going higher and you are in overdrive where the output of the transmission is revolving faster than the engine’s output.

Differential Gear Ratio

The gear ratio most interesting to 4×4 truck owners is the ratio between the driveshaft and axles – the differential gear ratio or axle gear ratio. It is controlled by the number of teeth on 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.

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The correct axle 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:

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 Axle Gear Ratio Formula

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

This formula is for 4×4’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.

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 4×4 Differential Gear Ratio Chart Table

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

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

Get your ring and pinion gear ratio today!

More gear ratio, tire information and some lifted trucks!

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