Gear Ratio Chart

The gear ratio most interesting for 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.

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:

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.

Another formula

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

This formula is for 4x4's with manual transmission. It should also be fine for an automatic with 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 Gear Ratio Table