Do Bigger Tires Affect Speed, Odometer and Gear Ratio? [Fully Answered]

As a mechanic, I have seen many car enthusiasts asking whether bigger tires affect their vehicle’s speed, odometer, and gear ratio. It’s a question that generates a lot of debate, with some saying that bigger tires will increase your vehicle’s speed and others claiming the opposite. In this article, I will dive into the nitty-gritty of tire size and its effect on odometer or speedometer readings to help you understand whether bigger tires are right for your vehicle. 

So, do bigger tires affect speed? While larger tires may look impressive, they can have an effect on your vehicle’s speed and odometer readings if the outer diameter of the new larger tire also increases. A larger tire with a greater outer diameter than the factory tire will have a greater circumference, due to which it will turn slower but cover the same distance. Due to this reason, the actual vehicle speed will be the same. In vehicles, odometers and speedometers are calibrated based on the size of your original factory tires. Larger tires will make the speedometer read slower than the actual speed. As a result, large tires will affect speedometer reading. Similarly, with a bigger tire, each rotation covers more ground, resulting in fewer rotations needed to travel the same distance. This means the odometer will read less than the actual distance traveled with a bigger tire. So, you need to calibrate the speedometer and odometer after installing the larger tires.

Also Read: Do bigger tires affect mpg

Some Importance Dimensions Of a Tire

The size of a tire is usually found on the sidewall of the tire and is represented by a combination of numbers and letters. For example, 175/65R15 84H. The numbers that follow represent the width of the tire in millimeters (175), the aspect ratio of the tire (65), which is the ratio of the height of the sidewall to the width of the tire, and the diameter of the wheel in inches (15). The wheel diameter is the same as the inner diameter of the tire.

If tire width is increased, the section height of a tire should also increase to maintain the same aspect ratio.

You can use this tool to play with the different dimensions of the tire and their effect on the tire revolutions.

The load index and speed rating of a tire indicate the maximum weight and speed that the tire can handle safely. These dimensions are critical to your car’s safety and performance. Overloading a tire or driving at speeds higher than its rating can increase the risk of a blowout or other serious accidents. In the above picture ‘84‘ is the load rating and ‘H‘ is the speed rating. You can consult this load and speed rating chart to learn more.

Understanding Of Tire Circumference

Tire circumference is the total distance of one revolution of a tire. The circumference of a tire can be calculated using the formula:

C = 2πr

where C is the circumference, π (pi) is a mathematical constant approximately equal to 3.14, and r is the radius of the tire.

Now, many people confuse tire radius with wheel radius. In the above formula of the circumference of the tire, the radius ‘r’ refers to the outer radius of the tire.

The radius of a tire is the distance from the center of the wheel to the outer edge of the tire.

The tire radius is mathematically calculated as follows:

r = wheel radius + section height

In the example at the start of this article, ‘15 in‘ was the wheel diameter. So, its radius will be half i.e. 7.5 inches. 

Aspect Ratio of the tire/100 = Section height/Width of the tire

Since AR of the tire was 65 in the above example and the width was 175mm, section height = 65/100 x 175 = 113.75 mm =  4.4783 inches.

Now, tire radius can be calculated as r = wheel radius + section height = 7.5 + 4.4783 =  11.9783 inches.

Now, we can calculate its circumference as follows:

C = 2πr = C = 2 x π x 11.9783 = 75.3989 inches.

This means that for every full rotation of the tire, the vehicle will travel a distance of 75.3989 inches. This is the same as if you cut the tire into a straight piece of rubber, its total length will be equal to its circumference in a circular shape.

Tire circumference is an important factor to consider when changing the size of the tire. As we saw earlier, a larger tire will have a greater circumference than a smaller tire. This can have an impact on the vehicle’s performance, including its acceleration, braking, and handling characteristics.

How Do Bigger Tires Affect Speed?

When it comes to the relationship between bigger tires and speed, it’s important to understand the difference between linear speed and rotational speed. Linear speed refers to how fast an object is moving in a straight line, whereas rotational speed refers to how fast an object is spinning around an axis.

Rotational speed refers to the number of rotations made by a wheel or tire per unit of time. It is commonly measured in revolutions per minute (RPM). On the other hand, linear speed is the distance covered over time. For example, if a car travels 60 miles in 1 hour, its linear speed would be 60 miles per hour.

With bigger tires, the linear speed of the vehicle remains the same because the distance the vehicle covers with each rotation of the tire stays constant.

This is due to the increase in circumference, the tire has to rotate fewer times to cover the same linear distance. This decrease in tire rotation results in a decrease in rotational speed, while the linear speed remains the same. 

We can demonstrate this relationship using a simple formula where the linear speed (V) of a vehicle is equal to the tire’s rotational speed (R) multiplied by the outer radius of the tire. 

V = R x C/2π

If we assume a vehicle is traveling at a linear speed of 60 mph (miles per hour) with tires that have a circumference of 75.3989 (calculated in the previous section), the tire’s rotational speed would be:

R = V / C  = 60 mph x 2π/ 75.3989 inches ​≈ 840 RPM.

Note: Here, I have not completely shared the calculations as the blog post will be more complex. Being an engineer, I want to just keep things simple. So, I thought I shouldn’t show calculations. In the background, I converted them into the same units i.e. inches to miles. Then, I converted the final result, which was in radians/hour into RPM. 

Now, if we increase the tire size to one with a radius of 14 inches, its circumference will be 87.92 inches. The vehicle’s linear speed will still be 60 mph but the rotational speed of the tire will decrease.

R = V / C  = 60 mph x2π/ 87.92 inches ≈ 720 RPM

So, you can see that the RPM of bigger tires is actually reduced. In vehicles, this RPM is determined by the vehicle speed sensors. 

After that, the control unit processes the speed of the tires (RPM) to calculate the vehicle speed in mph, which moves the needle to display the vehicle speed.

Technically, linear speed i.e. vehicle speed remains the same with both original factory tires and bigger tires. This speed doesn’t depend on the size of the tires.

However, the control unit in vehicles processes RPM into linear speed by taking into account the tire size. Since, the tire size given as the input by the manufacturer is the factory tire size, the linear speed will be displayed less.

So, if the wheel speed determined was 720 RPM, and the control unit of the vehicle considered the previous tire size i.e. 11.9783 inches (75.39 inches of circumference), the linear speed shown by the speedometer of your vehicle will be 51.306 mph. 

How Do Bigger Tires Affect Odometer?

Larger tires cover more ground with each rotation, which means your odometer will register fewer miles than you have actually driven. Keep in mind that the odometer shows linear distance but it calculates it from the angular distance. I’ll explain later in this section.

To calculate the impact of larger tires on your odometer readings, you can compare the circumference of the old tires with the new tires. You need to calculate the ratio of the circumference of the old tire to the circumference of the new bigger tire. Then, multiply the ratio by the distance covered by the vehicle with the original tires. 

Angular distance of a new tire = (Old tire circumference / New tire circumference) x Angular distance of an old tire

Just like the angular speed and linear speed discussed in the previous section, a tire (or any circular shape) has an angular distance (θ) and a linear distance (S).

To explain the angular distance for you, I have created an illustration below:

In the above picture, you can see that after some time, the point on a tire (red dot) moves from position 1 to position 2. That distance along the circumference of the circular shape of the tire is called angular distance. It is expressed in terms of the ‘radians’ unit.

Now, let’s say your tire has a radius (r) of 12 inches and it has to cover a linear distance (S) of 3 miles.

The mathematical relation between angular distance and linear distance is as follows:

θ = S/r

To make a calculation, ‘S’ and ‘r’ should be in the same units. So, convert 12 inches to miles, which becomes 0.000189394 miles.

θ = 3/0.000189394 = 15,844 radians ≈ 2521 revolutions.

So, the above result shows that a tire with a radius of 14 inches will have to rotate 2521 times to cover a distance of 3 miles.

In the previous sections, I calculated the circumferences of 12 inches and 14 inches tire sizes as 75.39 inches and 87.92 inches respectively.

So, the angular distance by a 14 inches tire to cover the same linear distance of 3 miles is:

θ = 75.39/87.92 x 2521 = 2161 revolutions. 

Now, 

Assuming the old tires have a radius of r1 and the new, bigger tires have a radius of r2, the angular distance covered by the old tires is calculated as:

θ1 = s / r1

And the angular distance covered by the new, bigger tires is calculated as:

θ2 = s / r2

The impact on odometer reading due to a bigger tire will be a percentage difference between θ1 and θ2 can be calculated using the following formula:

% impact on odometer reading = (θ2 – θ1) / θ1 x 100 = (2161-2521)/2521 x 100 = 14.28%.

This means that the odometer reading with the new, larger tires would be 14.28% lower than the actual distance traveled.

Just like linear speed, technically, a vehicle covers the same linear distance as well. 

The odometer is controlled by the engine control unit (ECU) which receives input from various sensors including the speed sensor. The speed sensor measures the rotation of the tires and sends this information to the ECU. The ECU then uses this information to determine the linear distance traveled by the vehicle and displays it on the dashboard.

But since the engine controller has an input of old tire size, it will not process the correct linear distance and shows a lesser linear distance than the correct linear distance covered by the vehicle.

How Do Bigger Tires Affect Gear Ratio?

A too much larger tire can alter the gear ratio to a significant extent, making your vehicle feel sluggish or slow to accelerate. When you add larger tires to your vehicle, you essentially increase the overall diameter of the wheel and tire combination. This, in turn, increases the distance that the wheel travels with each rotation. As a result, your vehicle will travel a greater distance with each revolution of the tire. This change in distance can affect your gears’ ability to transfer power to the wheels effectively. 

A gear ratio refers to the relationship between the number of teeth on two gears that are meshed together. This relationship determines how many rotations of the input shaft are required to make one rotation of the output shaft. If the gear ratio is too high, your vehicle may struggle to accelerate, and you may experience a reduction in fuel efficiency. 

Here, the gear ratio is basically the axle ratio. It refers to the ratio between the number of revolutions of the driveshaft and the rear axle. This is how a vehicle axle looks:

Source: https://club.mobilindustrial.com/lube_talk/b/tip_of_the_week/posts/part-one-differential-and-planetary-gear-units

The drive shaft of the transmission has a pinion gear that transfers the power to the ring gear. From the ring gear, power is transferred to the side gears, which rotate the tires of the vehicle. 

The ratio between the pinion and ring gear is called the axle ratio or gear ratio. For example, if the gear ratio is 4:1, the drive shaft will rotate four times to make the axle shaft turn once. 

Once the tire size is changed, the effective gear ratio also changes as the tire is also acting as a gear.

To understand this better, let’s consider two different scenarios. In the first scenario, we have a vehicle with a stock tire radius of 12 inches and a gear ratio of 3.73. In the second scenario, we upgrade to a tire radius of 14 inches. 

To calculate the new gear ratio, we can use the following formula:

New Gear Ratio = (Old Gear Ratio x Old Tire Diameter) / New Tire Diameter

Using this formula, we can calculate the new gear ratio as follows: New Gear Ratio = (3.73 x 12) / 14 = 3.197

From this calculation, we can see that the gear ratio has effectively changed from 3.73 to 3.197. This means the engine will have to work harder to turn the larger tires, resulting in a loss of torque and acceleration. Due to this reason, fuel consumption also increases when you shift to bigger tires.