Gear Ratio Chart Explained — Find the Best Ratio for Your Tire Size

Do you have a car and want to match power and fuel economy perfectly? Would you like to understand the connection between the gear ratio and tire size? So, you are in the right article. We are here to show you how to read and understand the gear ratio for tire size chart.

In principle, the gear ratio chart looks like a big and tedious table. However, it informs you how the tire diameter and gear ratios work together to determine the engine RPM, performance, and fuel economy.

Along with this article, we are going to explore the following subjects in detail:

• What is a gear ratio for tire size chart?
• How to read a gear ratio chart.
• Formula for tire size vs. gear ratio charts.
• Do larger tires lower the gear ratio?
• What is the best gear ratio for highway driving?
• What is the best gear ratio for 37-inch tires?
• And much more.

So, start your engine and let's explore the gear ratio chart.

Do you know how fast your car can go? Well, the answer to this question depends on many factors, but two components are essential to giving you this information: the gear ratio and the tire size.

The gear ratio is defined as the fraction between the circumference of the output gear (known as the ring gear) and the circumference of the input gear (also called pinion gear) in a gear train. Therefore, it is a parameter to measure the mechanical advantage of a given machine. If you want more details about the gear ratio, access our excellent gear ratio calculator.

The tire size (or its diameter) is also an important parameter to define your vehicle's performance. As you can imagine, if your tires have a larger diameter, they will travel farther with each revolution, which reduces your effective gear ratio.

This is why, in an off-road vehicle, if you switch your tires to 37-inch or 40-inch, you will need to re-gear the axle. Thus, the appropriate gear ratio adjustment is necessary to keep your vehicle performing well and to avoid burning excess fuel.

The question then is how to properly adjust the gear ratio and the tire size for your needs. You can do a perfect match by understanding the tire size vs. gear ratio chart, which is presented below:

In the axes of this chart, we can see the tire diameter in inches versus different gear ratios. The numbers presented in the cells are the values of the RPM (rotations per minute) at $65\,\mathrm{mph}$ or $105\,\mathrm{km/h}$, considering a manual $1:1$ transmission gear. Moreover, the blue cells represent a fuel economy mode, the green cells are configurations suitable for best performance, and the yellow cells represent the maximum power set.

The following section will present the details about correctly reading the gear ratio chart.

You can easily read a gear ratio for tire size chart by following the steps below:

1. Find the diameter of your tire, and for that you can use our tire size calculator.
2. Take the proper value of your gear ratio. You can find this value on the door tag or on your vehicle's axle.
3. Check out the value for your engine's RPM for this specific configuration. Remember that this considers a speed of $65\,\mathrm{mph}$ with a manual $1:1$ transmission gear.
4. You can also use the chart to verify new configurations for your car, prioritizing higher final speeds, balancing speed and acceleration, or searching for maximum acceleration.

By looking at the gear ratio chart, we can see, for instance, that a car with a tire of $28\mathrm{''}$ will be in a fuel economy mode for gear ratios between $3.07 \text{\textendash} 3.42$.

Now that you understood the tire size vs. gear ratio chart, let's see how we can compute these values given the $\mathrm{RPM}$.

The $\mathrm{RPM}$'s presented in the gear ratio chart were derived considering the formula:

where:

• $\mathrm{RPM}$ — Rotations per minute of your engine;
• $v$ — Speed of cruise, usually chosen as $65\,\mathrm{mph}$; and
• $336.13$ — Conversion factor from miles/hour to inches/minute.