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Big Tires, Fast Speedo? Unveiling Truths!

When considering the impact of bigger tires on your vehicle, it’s crucial to understand the mechanics behind a speedometer. Essentially, a speedometer measures the distance travelled in relation to tire rotation. With larger tires, the circumference increases, meaning for every revolution, your car covers more ground. This discrepancy can make your speedometer read slower than your actual speed. For instance, while you may be keeping pace with 55 mph traffic, your speedometer could be showing a lower speed due to the larger diameter of your tires compared to the original ones specified for your car.

A practical way to gauge this effect is to use a GPS or a navigation system like the Global Positioning System. These devices calculate speed over time, offering an accurate reading of your true speed. This method is especially useful when the speedometer is affected by tire size changes. In essence, while your speedometer might indicate one speed, the actual speed of your vehicle could be faster, particularly if you’ve switched to larger diameter tires. It’s like putting tractor tyres on your car; they rotate more slowly, affecting what your speedometer registers. To ensure you’re not unintentionally speeding or going below the legal speed limit, it’s advisable to match your speed with the flow of traffic, regardless of what your speedometer shows.

How a Change in Tire Size Affects Speedometer Accuracy

When you change your tires or wheels to a larger wheel, it’s essential to consider the overall diameter (OD) of your original tire. A lower profile tire often accompanies a larger wheel to keep the OD close to the original tire/wheel combination. This attention to detail is crucial, as even a 5% difference in OD compared to the original tire can significantly impact the gearing and correct operation of your vehicle. Notably, any change in the tire OD can lead to a false reading of speed and distance on your odometer.

Your vehicle’s speedometer is typically calibrated at the factory to match the exact size of the intended tires. If you opt for a taller tire, its circumference increases, meaning one rotation takes the vehicle further than with old tires. If the speedometer isn’t re-calibrated to reflect this change, it will likely register a slower speed than you’re actually traveling. For instance, a 3% taller tire, still within approved guidelines for plus fitments, would cause the speedometer to show 60 mph when you might be going 63.3 mph.

To measure your speedometer’s accuracy, conducting a road test is advisable. Use freeways with mile markers to determine the actual length of a mile you’re traveling. With a passenger handling a stopwatch and your car’s cruise control set at 60 mph, time your journey between mile markers. Ideally, it should take 60 seconds to reach the next mile marker. Repeat this test several times, and average the resulting times. If your average time deviates by more than 3 seconds, your speedometer likely needs re-calibration, especially if you’ve changed your tire and wheel package, affecting the revolutions per mile.


Some may cringe at this, thinking it’s a dumb question, but it’s quite valid. For example, if you’re offered 17’s and they fit, there’s still the question of how they affect the speedo. As the wheel travels further for a full rotation, it’s not just the rim size that matters, but the tire circumference. If you alter this, you alter speedometer accuracy. A larger wheel with the same wheel-tire circumference maintains an accurate speedometer. Conversely, a circumference smaller than stock spec makes the speedometer read faster than your actual speed, while a larger circumference would make it read slower.

In some cases, like with a Mustang that went from 15s to 17s, there’s no issue as long as the tires are sized accordingly. This means that larger wheels don’t necessarily mess up the speedometer. Some vehicles are easier to adjust for accuracy, with options available in a vehicle programmer for overall diameters ranging from 29″ to 44″. Others might require physical gears to be changed. This customization allows for maintaining speedometer accuracy despite changes in tire size.


When it comes to tire size, its impact on speedometer accuracy is significant. The concept of up-sizing or installing a taller tire can lead to a speedometer reading that is slower than your actual speed. This happens because the larger tire has an overall larger circumference, requiring it to travel more distance per revolution compared to the original equipment tire. Conversely, downsizing to a shorter tire will cause the speedometer to read a faster speed than you are actually going. This is due to the smaller tire having a smaller circumference, enabling it to travel less distance per rotation.


When you change your tire size, it’s crucial to recalibrate your speedometer to safely operate on the road. This ensures you are not accidentally speeding or driving dangerously below the flow of traffic speed. While Discount Tire does not offer recalibration services, they recommend visiting a professional auto shop for this task. For drivers of modern cars, recalibration can often be done through the calibration settings on the vehicle’s computer. This involves driving a prescribed distance to adjust the settings. However, for safety reasons, it’s better to consult professionals. Ask your Discount Tire technician to refer you to an appropriate shop for recalibration.


When it comes to understanding how tire size affects your speedometer, it’s crucial to grasp the basics of speed calculation. Typically, a speedometer operates on the principle of measuring the number of tire rotations over a given distance. Now, if you’ve manually switched to larger tires, your speedometer might not calculate your speed accurately. This is because a larger tire will cover more ground per rotation compared to a smaller one.

To get a grip on this, you can use road mile markers as a reference. By using a stopwatch to track the exact number of seconds it takes to travel the distance of a single mile, you can convert this data into the mile per hour metric. However, remember this method might not always be the most accurate.


For more precise results, consider using a tire size and speedometer calculator. Alternatively, modern solutions like a GPS device or a mobile mapping app with a satellite speedometer function can offer real-time speed measurements, bypassing the limitations of traditional speedometers.

How much will my speedometer change with bigger tires?

Ever wondered how a change in tire size impacts your speedometer’s accuracy? Let’s dive into this with a bit of personal experience and expertise. Picture this: you’ve just swapped your old tire, with a circumference of 60″, for a new tire boasting a 66″ circumference. This upgrade is more than just a visual change; it significantly affects your speedo-indicated speed.

To understand this, consider the ratio between the circumference of your current tires and the larger tires. In our example, this ratio is larger than 1, indicating that your new tires are making more ground with each revolution. This means that when your Speedo reads 55, your actual speed might be higher. But how much exactly?

The math here is surprisingly simple. For every odo-indicated mile, the odometer should read as if you’ve traveled 1.0 miles, but with larger tires, you might actually cover, say, 0.9 miles. This size difference introduces an inaccuracy. To calculate the speed error, you can use road markers and time your travel between them at a given time, comparing it with the Speedo and odo-indicated readings.

This isn’t just about every speed; it impacts your odometer reading too. Over time, this difference in diameter and circumference can lead to a significant divider in the number of miles your car reports having driven. Such a discrepancy not only misinforms but can affect things like your vehicle’s resale value or warranty coverages.

So, when contemplating switching to larger tires, remember this: a speed error isn’t just a number on your dashboard; it’s a reflection of the difference in how your car interprets its travel.

Why is it that as the tires wear down, the more inaccurate the speedometers become?

As a car enthusiast, I’ve always been fascinated by how tire wear affects speedometer accuracy. The speedometer essentially measures the number of wheel rotations over time to calculate your car’s approximate speed. Since the size of your wheel, specifically its radius and circumference, is proportional to the indicated speed, any change in these dimensions directly impacts accuracy.

With tires wearing down, there’s a decreasing radius, leading to a smaller circumference. This decrease may seem negligible, but it has a measurable impact. Consider a small, 16” wheel with a 205/50 section tyre. Initially, the circumference might be around 300mm. However, a 6mm loss in tire material, which is around the legal limit, results in about a 2% difference in circumference. What does this mean? Your speedometer starts reading faster. So, when you think you’re driving at 100kph, you might actually be around 1.2mph slower. At 60mph, this difference becomes more pronounced.

This diameter change not only affects the speedometer but also the entire drivetrain. It influences the engine’s revolutions per minute at the same speed, potentially impacting fuel economy. The impact of circumference changing is a subtle yet vital aspect of vehicle maintenance, reminding us that even the smallest components play a significant role in the bigger picture.

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Does changing to bigger tires on a vehicle make the speedometer or odometer a little “off”?

Switching to bigger tires on your vehicle can indeed throw your speedometer and odometer readings a bit “off.” Let’s delve into why. The speedometer and odometer are calculated based on the circumference of your tire/wheel combo. Installing a larger tire changes the car’s wheel revolution per mile, affecting both speed and distance measurements.

Consider a jump from a 16-inch wheel to an 18-inch wheel. Along with this, there’s often a change in sidewall height of the tyre to maintain the overall diameter close to the original tyre. Even though this helps in maintaining some level of speedometer accuracy, it’s not always perfect. For example, switching from the stock base model wheels of a Commodore or Chevy Lumina Omega, say from 205×65 x 15″ wheels to 225 x 50 x 16″ wheels, alters the dynamics.

The new setup with lower profile rims and wider tyre has a significant impact on the sidewall profile. Even a small percentage change in width or sidewall height can cause discrepancies in actual mileage readings and speedometer display. It’s a trade-off many car enthusiasts make for aesthetics or performance, but it’s important to be aware of these changes and compensate accordingly, especially when comparing the wear of a worn out tyre to new tyres.

Does changing the tire size change your speedometer?

Yes, changing your tire size can indeed affect your speedometer’s accuracy. As someone who drove a Jeep with bigger tires, I learned this first-hand. A fellow radio operator and I were following each other on a trip, and as we talked back and forth, we noticed something intriguing about our speed readings. Despite driving at the same pace, my speedometer was consistently showing about 5 MPH off compared to theirs.

This difference was because the larger tires on my Jeep had a greater circumference, causing the speedometer to under-register my actual speed. So, every time my speedometer showed 60 MPH, I was actually traveling slightly faster. This discrepancy is a common phenomenon for vehicles with modified tire sizes, and it’s something to keep in mind for anyone considering an upgrade.

How do you fix a speedometer when changing tire size?

Back in the early 70s, at our family car dealership, we often dealt with police cars from the town that had larger tires installed. This screwed up their speedometers, and they needed to be fixed. The solution involved changing a small gear connected to the speedometer cable. I vividly remember the process: replacing the smaller gear with a larger gear was typically a matter of trial and error to get the speedometer reading correct.

This method, although somewhat rudimentary, effectively compensated for the changes in tire size. Each time the tire size altered, the gear ratio needed adjustment to ensure that the speedometer accurately reflected the vehicle’s speed. It was a straightforward yet crucial fix, essential for maintaining the accuracy and reliability of these important vehicles.

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