Tyre upsize guide: How to calculate tyre size for your car?

Team AckodriveJun 6, 2022

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Just like your outfit enhances your looks, a car's wheels and tyres enhance its appeal. That's the reason why you see so many cars on the road with aftermarket wheels and tyres. Such modifications involve upsizing the tyres or wheels. But if not done correctly, it may harm the car's efficiency and performance. That's why having proper know-how of car tyre upsize is helpful during such modifications. If you plan to upgrade your car's tyres, this article provides deep insights into the tyre upsize guide and wheel size calculator.

Tyre upsizing

What is tyre upsizing?

Tyre upsizing is nothing but installing wider tyres in your car to get a bold stance and improve the overall road grip. Tyre upsizing can also be done by installing larger wheels (alloy wheels) along with a wider tyre. Both the methods will lead to the same result, sporty look and improved handling capabilities due to the better grip.

Now, you may think about how car tyre upgrades can improve handling? When you install wider tyres, their contact patch with the road surface increases. Hence, the car's weight is distributed efficiently, and the car feels stable on the road, especially at high speeds and cornering. 

However, if you end up miscalculating the tyre size for your car, then it may lead to serious issues related to performance, safety and fuel consumption. We will touch upon those factors in the following sections.

Tyre upsize calculator for car: How to calculate tyre size?

Tyre upsize calculators will come in handy if you wish to upgrade your car tyres. Your car's tyre size plays a crucial role in the vehicle's optimum performance. If there's an issue with the tyre size, the performance and fuel efficiency will take a hit, and you do not want that to happen. That's the reason why you should have appropriate knowledge of tyre upsize calculation before venturing into the world of this type of modification.

This section explains the basic rules that you should follow while modifying your car tyres/wheels.

1. Upgrading to wider tyres while retaining the original wheels

In this type of modification, you only change the rubber tyres. There's no need to fiddle with the metal portion; that is the wheels. You can install a wider tyre as per your requirement and the fitment possibility/compatibility.

To be safe while swapping the tyres, always ensure that the new tyres' Overall Diameter (OD) remains within the permissible limits. That is up to 3 percent of the stock tyre's original diameter.

Example:

Stock tyre diameter: 24.47 inches

New tyre diameter (Permissible limit): 25.20 inches

To understand it even better, let's look at this example where the original tyre size stands at 185/65 R15. Here, the original diameter of the stock tyre is 24.47 inches.

Calculating tyre width

Stock tyre width: 185 mm

Safe upsize limit: Add 20 mm to the stock tyre width.

New tyre width: 205 mm

Calculating tyre sidewall height

Stock tyre sidewall height: 65 percent of the width

Note: For every 10 mm increase in tyre width, the sidewall height needs to be reduced by five percent.

In this example, we increased the tyre width by 20 mm. Hence, the sidewall height can be reduced by 10 percent.

New tyre sidewall height: 55 percent of the width

Wheel/rim size

Since we are not modifying the wheels, the size remains the same; that is, 15 inches.

So, the maximum upsize limit for a stock tyre of 185/65 R15 would be 205/55 R15.

2. Upgrading to larger wheels/alloy wheels

This type of modification involves replacing the manufacturer-provided stock wheels. Upgrading to bigger size wheels enhances the overall appeal of the car. Generally, the tyres for the larger wheels are known as low-profile tyres as the tyre sidewall height is on the lower side. The tyres are fitted as per the requirement or depending on the space between the wheels and the wheel arches.

When upgrading to a bigger wheel, you still need to follow the permissible limit mentioned in the previous example.

If you wish to upgrade to a 16-inch wheel, the new tyre size would be 205/55 R16. Remember that the original diameter should be within the permissible limits (3 percent); that is 24.47 to 25.20 inches.

Tyre upsizing benefits

Below are the advantages of upgrading your stock car tyres to wider tyres.

  • Better road grip: As mentioned earlier, switching to wider tyres increase road grip. Due to the efficient weight distribution, the car feels planted on the road when you corner or during high speed runs. Wider tyres give you a lot more confidence and control over your car, which is crucial while driving at high speeds.

  • Improved cornering stability: One of the significant advantages of tyre upgrading is the vehicle's enhanced stability or overall balance. With wider tyres, the contact patch increases, and with a broader contact patch, the overall stability of the car increases. The wider contact patch and lower sidewall aid in better handling.

  • Improved braking: Having confidence in your car's brakes is crucial for a safe drive. Wider tyres grip the road surface better than stock tyres. It results in a shorter braking distance. Grippier tyres give you a lot of confidence while braking. Hence, you can control the vehicle even in situations where you have to slam hard on the brakes.

  • Superior aesthetics: One of the reasons for tyre upgrading is the superior exterior looks. Wider tyres and larger wheels render a sporty look to the car. With an infinite number of alloy wheel designs on the market, you can personalise your car as per your requirement.

  • Weight reduction: You can upgrade to a lighter wheel while upsizing the wheels. The weight reduction will improve the car's handling and overall performance characteristics.

  • Improved fuel economy: The weight reduction may also lead to improved fuel economy, especially if you regularly drive in stop-and-go traffic.

Also, read: How to get the Maximum Fuel Efficiency from your Car?

Drawbacks of tyre upsizing

Here are some of the demerits of car tyre upsizing.

  • Reduced fuel economy: As you already know, wider tyres grip the road surface better than stock tyres. It may have a positive impact on the handling of the vehicle, but it will have a negative effect on the fuel economy. Wondering how? Increased road grip means increased rolling resistance, which means the engine has to work harder. The harder the engine works, the more fuel consumption. It’s a chain reaction that leads to reduced fuel economy.

  • Heavy steering: Installing wider tyres or bigger wheels may lead to a heavy steering feel. The added weight of the tyres/wheels may impact the steering precision and can have a negative impact on the overall handling of the vehicle.

  • Harsh ride quality: Generally, upsized tyres have a lower sidewall, which translates to a bumpy riding experience. That’s because the lower the sidewalls, the less effective it is in absorbing bumps. That’s the reason why you don’t see low-profile tyres on mass-market cars.

  • May damage the wheel arches: An upsized tyre may stick out of the wheel well or the wheel arch. Aesthetically it may look good, but they end up hitting the wheel arch when driving over the bumps or potholes.

Does tyre size increase ground clearance?

Yes, increasing the tyre size will increase the ground clearance, provided you also increase the wheel size. Ground clearance is nothing but the distance between the road surface to the lowermost part of the car. One of the apparent outcomes of increasing the tyre and wheel size is the increased ground clearance. However, if you only upgrade the tyre, you cannot expect a significant increase in the ground clearance.

Also, read: Top cars with highest ground clearance in India

Does changing tyre size affect transmission?

Changing tyre size does not affect the transmission. However, wrong size tyres may have an impact on the final drive. For example, if the tyres are too large, the final drive ratio will be low. Similarly, too small tyres will result in a higher final drive ratio. But the effect is very menial as it is not possible to install too large or too small tyres unless you are into some extreme car modifications.

Frequently Asked Questions (FAQs)

Below are some of the most common queries regarding car tyre upsize.

Can I upsize my car tyres without changing the rims?

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Yes, you can change the tyre size without changing the rims or wheels. But make sure that the tyre and the rim have the same internal diameter. The tyre will not fit into the rim if the diameter doesn't match.

Is tyre upsizing a legal car modification in India?

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Yes, changing the tyre size is a legal car modification in India, provided you are within the limitations set by the manufacturer. Also, ensure that the larger tyres have the same or higher speed rating and load index than the stock tyres.

Do bigger wheels damage my car?

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If you switch to larger wheels, it can cause suspension and brake-related issues if the tyre sidewall does not suit the vehicle. Even with the low-profile tyres, the chances of blowouts are high.

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