Can You Change the Middle Number on Tires: Possibilities and Impacts

By Arif

Tire Tread

One of the most commonly misunderstood aspects of car tires is the series of numbers on the sidewall, such as 205/55R16. Many motorists consider using different numbers of tires. But can you change the middle number on the tires?

The series of numbers and letters might seem alienated to the common users however, these markings contain vital information about your tire’s size, load-carrying capacity, speed rating, and much more.

In this informative blog post, we will help you understand tire label readings and how changing them could impact the performance of your vehicle.

Can You Change the Middle Number on the Tires?

Yes, you can change the middle number on tires with certain conditions. The middle number, the aspect ratio, is a crucial factor in maintaining the overall diameter of the tire. It represents the height of the tire’s sidewall as a percentage of its width.

For example, in the tire size P215/65R15, the 65 (as the middle number) means that the sidewall height is 65% of the tire’s width, which is 215 millimeters. Changing this number can affect your speedometer readings, which means impacts on your car’s handling.

If you change the middle number or the aspect ratio, it could lead to inaccurate speed readings and affect the accuracy of your car’s odometer.

For example, if you go to a lower aspect ratio, your tires will have a stiffer sidewall, which may improve handling but reduce ride comfort. This scenario will change adversely if you go to a higher aspect ratio.

Furthermore, altering this ratio might also cause the tires to rub against the fenders or other parts of the vehicle, which can exaggerate the damage to potential accidents.

If you want to change the middle number of your car tire, ensure to keep the overall diameter of your tires the same meaning you will also need to change the width of your tires along with the changed aspect ratio.

If the aspect ratio you are choosing is lower than the current one, you will need to go for a wider tire to maintain the same overall diameter.

Moreover, do not change the aspect ratio for any one tire. Instead, change the aspect ratio on a pair of tires or all four tires. Also, consider using the changed aspect ratio tires on the rear wheels rather than driving wheels for optimum safety and performance.

If you are unsure whether or not it is safe to change the aspect ratio of your tires, it is always best to consult with a tire expert or learn how to read tire labels before choosing the right size tires for your vehicle.

Understanding Tire Labels: How to Read the Numbers on the Sidewalls of Tires

To decode the numbers and letters on your tire, let’s break down our example, 205/55R16:

Tire with Numbers

205 (Tire Width) – The first number, 205, indicates the width of the tire from sidewall to sidewall in millimeters. It measures the distance from one sidewall to the other. In this case, the tire is 205 mm wide, which is approximately 8.07 inches.

55 (Aspect Ratio) – The second number, 55, represents the aspect ratio. As mentioned earlier, this is the height of the sidewall from wheel to tread relative to the width of the tire measured in percentage. In this case, the tire’s height is 55% of its width.

R (Construction Type) – This indicates the construction of the tire. ‘R’ stands for radial construction, which is the most common type in modern tires. Radial tires have ply cords that run perpendicular to the tread, offering better stability and fuel efficiency.

16 (Wheel Diameter) – This is the diameter of the wheel in inches that the tire is intended to fit. In this example, the tire is designed to fit a 16-inch wheel.

91 (Load Index) – The load index is a two-digit number found next to the tire size. It indicates the maximum weight the tire can support when properly inflated. The higher the load index, the more weight the tire can carry.

The load index is calculated per tire which means 91×4. So, if your vehicle weighs 4,000 pounds, each tire with 91V can carry up to 1323 pounds without risking blowouts.

V (Speed Rating) – This represents the maximum speed the tire can handle safely. It is denoted by a letter (such as V, W, Y) and corresponds to a specific speed in miles per hour. For example, W indicates 168 mph, and Y indicates 186 mph while in our case V indicates a maximum speed of 149 mph.

Other Important Tire Information

Apart from the size, there are other markings on the tire that convey essential information:

Tire with Treadwear Label

Treadwear, Traction, and Temperature Grades – These grades provide information about the tire’s tread life, traction performance, and temperature resistance, respectively.

Treadwear is a three-digit number referring to how long the tire’s tread is expected to last. Higher numbers suggest a longer lifespan. Similar metrics apply to traction and temperature grades.

AA stands for the best and C stands for the worst ratings for traction on the road. Temperature ratings range from A (highest) to C (Lowest) to dissipate heat at high speeds.

Tire with DOT Number

DOT (Department of Transportation) Number – You will also see a 12-digit number on the tire’s sidewall. This is known as the DOT number. It represents the date of manufacture, with the first two digits indicating the manufacture week and the remaining two as the manufacture year.

Depending on this information, you can assess the tire’s age and its newness. In our example (1221), the week of manufacture is 12 and the year is 21.

Impacts of Changing the Middle Number on Tires

Now, that you have understood the information and logic behind tire numbers what happens when you practically change it meaning installing a different aspect ratio of tire than the original ones? Here are some of the impacts on your vehicle’s performance:

Speedometer and Odometer Accuracy

The aspect ratio is directly related to the overall diameter of the tire. So, if you change the aspect ratio, it will affect the tire’s height from wheel to tread relative to its width. This alteration can lead to inaccurate speedometer readings.

The speedometer and odometer of a car are linked with the dimensions of the tires. When your car’s tires have a different overall diameter than what the speedometer is calibrated for, it will display a speed that is either faster or slower than your actual speed. This discrepancy can lead to speeding tickets or even accidents due to misjudged speeds.

Similarly, the change in the overall diameter can also affect the accuracy of your car’s odometer, leading to incorrect mileage readings.

Handling and Performance

The aspect ratio also affects the handling characteristics of your vehicle. When you change the aspect ratio it will also change the sidewall stiffness and flex of tires. This will directly impact your car’s handling during the turns, bumps, and uneven road surfaces.

By changing the aspect ratio without proper consideration, you are decreasing your car’s stability levels, and compromising cornering abilities, which eventually leads to an overall less comfortable ride.

Tire Clearance and Potential Damage

What happens to the fenders and other associated parts of your car when you alter the aspect ratio?

Tires that are too tall might rub against the inner fender, especially when turning or going over bumps. On the other hand, tires with a smaller aspect ratio might not provide enough clearance, leading to contact with the vehicle’s suspension components.

Changing the aspect ratio can definitely cause the tires to rub against the fenders or other parts of the vehicle. This rubbing can cause damage to both the tires and the vehicle’s body or suspension, resulting in costly repairs.

Impact on Suspension and Braking Systems

Another impact due to changes in the aspect ratio can be on the suspension system’s geometry. An improper aspect ratio of tires can strain the suspension components and lead to premature wear and tear.

When the weight distribution goes uneven on the tires it can imbalance the vehicle’s handling, braking performance, and ride comfort impacting the overall safety and stability.

Fuel Efficiency

Want to pay more while fueling your car? Try altering the tire’s aspect ratio. Yes, altering the aspect ratio can also impact your car’s fuel efficiency.

Tires with different aspect ratios can change the rolling resistance, which affects how easily the tires roll along the road surface.

If the aspect ratios are significantly altered it might increase rolling resistance, leading to decreased fuel efficiency and higher fuel consumption.

Final thoughts

Changing the middle number on your tires can have significant repercussions on your vehicle’s performance and safety.

The numbers and letters on your car’s tires are not arbitrary; they convey vital information about your tire’s size, construction, and limitations.

If you are really considering changing the middle number or the aspect ratio, always adhere to the manufacturer’s recommendations or consult your vehicle’s manual or a tire professional.

This will ensure that any replacement tires you choose maintain the same size specifications as the original ones and keep your vehicle and its components unaffected.

Remember, the next time you glance at those numbers on your tire’s sidewall, they are telling you more than just the size – they are revealing the key to a safe and smooth ride.

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I am the founder of TheCarPlus. As a passionate car owner and enthusiast, I offer practical solutions and insights from my own personal experiences combined with extensive online research.

The purpose of creating this website is to empower fellow car owners and motorcyclists like 'YOU' with knowledge and practical solutions to enhance your vehicle's performance and aesthetics.

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