## Differential Gear Ratio Calculator

Our Differential Gear Ratio Calculator can help you select the correct gear ratio to maximize the performance of your vehicle. The addition of larger tires can greatly decrease performance and fuel economy. Choosing the right differential gear ratio will help regain lost power and torque.

- The Tire Diameter Calculator will allow you to determine your actual tire diameter.
- The Differential Gear Ratio Calculator or RPM Calculator will give you an estimate of your engine RPM, based on tire diameter and the gear ratio you select.
- The Gear Ratio Calculator will determine the ratio of a gear set, based on the number of teeth on the ring gear and teeth on the pinion gear. This is especially helpful when trying to identify a gearset that needs replacement with the same ratio ring and pinion.

## Tire Diameter Calculator

**START HERE!** You need to know your tire diameter in order to work with the RPM calculator relevant to gear ratio. Inaccurate tire size input will greatly affect the results from the Gear Ratio Calculator.

The calculated value should be used only as a guideline for RPM calculations. **The most accurate way to determine tire height is to physically measure from the ground to the top of the actual tire.** This dimension will “round up” slightly when the vehicle is in motion. Tires with identical size specifications that are made by different manufacturers may also have different actual diameters. **Wheel diameter does not affect gear ratio!!**

If you do not have access to measure the actual tire height, use the calculator to determine tire height based on the manufacturer’s size specifications. Tire specifications usually follow a three number format, such as 275/75R16, where 275 is the WIDTH of the tire tread in millimeters, 75 is the ASPECT (or percentage of tread width to yield the sidewall height), and 16 is the WHEEL diameter. Using the numbers in this example, you’ll get a tire height of 32.24″ by using the calculator below.

Truck and off road tires may have a specification like 30×9.5R15, where 30 is the tire diameter, 9.50 is the tread width and 15 is the wheel diameter. You don’t need the tire diameter calculator in this case.

#### Tire Diameter Calculator

## Engine RPM Calculator

Use the RPM calculator to determine what RPM your engine will run based on gear ratio, tire diameter, transmission and MPH. You can try various combinations to come up with an appropriate combination of ring and pinion gear ratio and tire diameter for your vehicle. The RPM calculator should be used as a guideline for comparative purposes only. Your vehicle will probably run at different RPM’s than those you calculate.

**Recommended Engine RPM @ Highway Speed**

- 4 cylinder: 2200 – 3200
- V6 cylinder: 2000 – 3200
- Small block: 1800 – 2800
- Big block: 1800 – 2600
- Diesel: 1600-2800

For a complete listing of the Ring & Pinion Gear Ratios available for your application, go to the **Sierra Gear Ring & Pinion Gear Sets** page.

#### RPM Calculator

Your RPM is about

Automatic without Overdrive:

Automatic with Overdrive:

Manual:

5 Speed:

## Gear Ratio Calculator

Identify the Ring and Pinion Gear Ratio you currently have by counting the teeth on the Ring and Pinion gears. Enter the number of teeth on your ring and pinion gear set in the calculator below to find the gear ratio, or simply divide Ring Gear tooth count by Pinion Gear tooth count (**Example:** Ring Gear / Pinion Gear = Ratio)

off road tires may have a specification like 30×9.5R15, where 30 is the tire diameter, 9.50 is the tread width and 15 is the wheel diameter. You don’t need the tire diameter calculator in this case.

#### Gear Ratio Calculator

## Calculating a "Return to Stock" Gear Ratio

After installing larger tires, it is advantageous to change the gear ratio to regain lost acceleration and fuel economy. The formula for determining an appropriate gear ratio is fairly simple.

Divide the New Tire Height (measured in inches or CM) by the Old Tire Height, then Multiply by the Original Gear Ratio:

New Tire Height / Original Tire Height X Original Ratio =** Return to Stock Ratio**

Example:

35″ / 31.9″ = 1.097 X 3.08 =** 3.37**

Notice that the new corrected ratio is 3.37, a ratio which doesn’t actually exist. This is merely your starting point. In most cases, the nearest available ratio will be 3.42 or 3.55.

**But**, there are a few other factors to consider before choosing the ideal ratio. Firstly, your new big tires probably weigh more than the old stock setup. Secondly, wider tires and a taller vehicle create more wind resistance. To compensate for this, you might want to choose a ratio that is at least one or two steps lower (higher numerically) such as 3.73 or 4.11.

Another factor to consider is how you intend to use the vehicle. For a primarily flat-ground high speed highway driven vehicle, the 3.73 or 4.11 might be just fine. For a vehicle that sees a lot of hills and more off road use, 4.56 or even 4.88 might be appropriate. Use our Calculators page to see how the different ratios affect engine RPM.