How to Compare Two Tire Sizes: A Step-by-Step Guide
Whether you are looking to change the look of your vehicle with a plus-size setup, seeking cheaper alternative tire sizes for winter driving, or lifting a truck for off-road use, comparing tire sizes is an essential skill.
But what exactly should you be looking at when comparing a factory tire to a new potential size? It is not just about the numbers on the sidewall; it is about how those numbers translate to physical dimensions and vehicle dynamics.
In this step-by-step guide, we will walk you through exactly how to compare two tire sizes, what metrics are most important, and the rules of thumb for ensuring a safe fit.
Step 1: Identify the Key Metrics
When comparing two tires, you are primarily looking at four key physical dimensions derived from the metric sizing code (e.g., 225/45R17):
- Overall Diameter: The total height of the tire.
- Section Width: The total width of the tire from sidewall to sidewall.
- Sidewall Height: The distance from the rim to the tread.
- Circumference: The total distance around the outside edge of the tire.
These four metrics will dictate how the new tire fits in your wheel well, how it rides, and how it interacts with your vehicle’s electronics.
Step 2: Check the Diameter and Circumference Difference
The most critical comparison between two tire sizes is the overall diameter (which directly correlates to circumference).
As we discussed in our guide on speedometer accuracy, changing the diameter alters the distance the tire travels per revolution. This affects your speedometer, odometer, anti-lock brakes (ABS), and transmission shift points.
The Rule of Thumb: When comparing a new tire size to your factory size for standard street driving, the new diameter should stay within ±3% of the original factory diameter.
For example, if your factory tire has a diameter of 25.0 inches, your new tire should ideally fall between 24.25 and 25.75 inches. Staying within this 3% window ensures that your speedometer remains reasonably accurate and your vehicle’s computer systems operate as intended.
Step 3: Analyze the Width Difference
The section width determines how much rubber is meeting the road and how much lateral space the tire takes up in the wheel well.
- Going Wider: A wider tire (e.g., moving from a 205mm to a 225mm) increases the contact patch. This generally improves dry handling, cornering grip, and braking distance. However, it can increase rolling resistance (lowering fuel economy) and increase the risk of hydroplaning in wet conditions.
- Going Narrower: Narrower tires are often chosen for winter setups because they cut through deep snow better than wide tires, which tend to float on top. They also offer lower rolling resistance.
When comparing widths, ensure you have physical clearance. A tire that is significantly wider may rub against suspension struts on the inside or the fender lip on the outside, especially when the steering wheel is turned fully.
Step 4: Evaluate the Sidewall Height
The sidewall height is the primary factor in ride comfort and steering response. When comparing sizes, note the difference in the sidewall.
- Taller Sidewalls: Provide a softer, more comfortable ride and better protection for your wheels against potholes. They are ideal for daily driving on rough roads or off-roading.
- Shorter Sidewalls: (Found on lower aspect ratio tires) offer stiffer construction, leading to sharper, more responsive steering and less body roll in corners. The tradeoff is a harsher ride.
Step 5: Check Wheel (Rim) Width Compatibility
Every tire size requires a wheel of a specific width to mount correctly and safely. A tire has an “approved rim width range.”
For example, a 225/45R17 tire might require a wheel width between 7.0 and 8.5 inches. If you are comparing your current tire to a new, wider size, you must ensure that your current wheels are wide enough to safely accommodate the new tire. If they are not, you will need to purchase new wheels as well.
Step 6: Verify Load Capacity
Safety should always be the top priority. Every tire has a Load Index (a two or three-digit number following the size, e.g., 225/45R17 91V).
When comparing tire sizes, the new tire must have a load carrying capacity equal to or greater than the original factory tire. Never install a tire with a lower load index than what your vehicle manufacturer specifies, as this can lead to catastrophic tire failure under heavy loads.
Using the Right Tools
Doing all the math by hand to compare diameter, circumference, width, and speed variances can be tedious and prone to error.
To make this process seamless, you should use our dedicated tire size calculator.
Our calculator allows you to input your current tire size (Size 1) and your desired new tire size (Size 2). It instantly calculates all the physical dimensions, provides a visual representation of the size difference, calculates the exact percentage variance in diameter, and tells you exactly how your speedometer reading will change.
By comparing the metrics carefully and staying within safe tolerances, you can confidently choose the perfect tire size for your needs, ensuring optimal performance, safety, and fitment.