How Does Tire Size Affect Speed?

How does tire size affect speed? In summary, larger tires can help achieve higher speeds but may slow down acceleration, while on the contrary, smaller tires improve acceleration. Changing your tire size can also affect your speedometer readings since most vehicles calculate speed based on tire revolutions, and altering the tire's overall circumference changes how the system interprets your actual speed. This can lead to your speedometer displaying a speed that's slightly higher or lower than the real one.

Effects On Your Actual Speed

Tire size has an impact on how your vehicle accelerates and reaches its maximum speed. When it comes to acceleration, larger tires generally add more rotational mass. As a result, your engine must work harder to get the wheels moving, which can lead to slower acceleration, particularly in smaller vehicles or those with lower horsepower. Smaller tires, on the other hand, are lighter and require less energy to rotate, allowing your vehicle to accelerate more quickly. In terms of top speed, larger tires can help you achieve a higher maximum velocity because your engine can run at lower RPMs to maintain that speed, but the increased weight means that it takes longer to reach it.

Effects On Your Speedometer

Most vehicles calculate speed based on tire revolutions, and when the vehicle was built, the system was calibrated for the original tire size. Installing a different size alters that relationship, which can cause your speedometer to display incorrect readings. A taller tire has a larger overall circumference, so it covers more distance per revolution. This makes your speedometer show a lower speed than you're actually traveling, and the difference becomes more noticeable as speed increases. A shorter tire does the opposite; it makes the speedometer read faster than your actual speed because each rotation covers less distance. For example, if your tires are 3% taller than the factory size, your speedometer may show 60 mph when you're really traveling about 63.3 mph. This is very important because if you are driving in an area with speed restrictions, let's say a school zone, you might not notice that you are actually travelling at a higher speed than what's permitted, and you will likely be pulled over by an officer.

How Do You Know How Inaccurate Your Speedometer Is?

Your speedometer should ideally be within 2 mph of your actual speed. Here are a few ways to check. You can always use a combination of testing methods to ensure accuracy.

1. Use a GPS - Many GPS devices and mobile mapping apps include a satellite-based speedometer feature. To test your speedometer's accuracy, drive on a flat, straight road with minimal traffic and obstacles and set a consistent speed. It's best to test at the highest legal speed you can, as this provides a more accurate percentage difference between your actual speed and what your speedometer displays.
2. Manual Calculation - Another simple method is to perform a 1-mile controlled road test: Use freeway mile markers to measure distance. Have a passenger use a stopwatch to ensure safety. Set your cruise control at 60 mph (or a similar consistent speed). Start the timer at one mile marker and stop it at the next. Repeat the test three or four times, then calculate the average of the resulting times. If your average time is off by 3 seconds or more, your speedometer may need recalibration. Here's a helpful table by the North Dakota Highway Patrol:

   Seconds	Miles Per Hour
   60	60
   61	59
   62	58
   63	57
   64	56

3. Online Tools - In addition to the methods mentioned above, you can use an online tool to calculate the difference in speed in your speedometer that changing your tire size can cause. We recommend this tool: Speedometer Error Calculator by inchcalculator.com.

How Can You Change Your Tire Size Without Affecting Your Speedometer?

Plus-sizing involves installing tires of different sizes while keeping the overall diameter and height unchanged. By keeping the overall dimensions consistent, plus sizing allows you to achieve a different look or feel without affecting your speedometer's accuracy.

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Frequently Asked Questions

Curious about how tire size affects speed? Here are a few common questions we get from drivers:

Do You Accelerate Faster With Smaller Tires?

Yes. Smaller tires are lighter and require less energy to rotate, which allows your vehicle to accelerate more quickly.

Do Bigger Tires Affect Your Speed?

Bigger tires can help your vehicle reach a higher top speed since your engine can run at lower RPMs to maintain that speed. However, they also add weight, which means that it usually takes longer to get up to that speed compared to smaller tires.

Do Bigger Tires Affect Your Speedometer?

A taller tire has a larger circumference, so your speedometer will typically show a lower speed than you're actually traveling. Even a small change can make a difference, for example, if your tires are 3% taller than your stock tires, a reading of 60 mph may actually mean you're traveling about 63.3 mph.