Does Tire Size Affect Speedometer Reading?
You often hear me talk about the complexity of cars and how they’re designed to operate in a certain way. Manufacturers fine-tune them during the development and manufacturing process so that you, as an end-user, get a perfect piece of machinery.
Whenever we drive our cars, we rely on multiple readouts that give us all the information about our mechanical friends. The needle for the fuel tank, the temperature gauge, the speedometer, and many other things. With modern cars, we get a lot more, thanks to the wonders of technology and sensors.
To ensure that things operate as intended, manufacturers design cars so that multiple components work in unison. Take the speedometer, for example. It shows us the speed at which we’re driving, but most people don’t bother to understand how it works.
Speedometers rely on multiple factors to give you a precise readout, and one crucial thing is tires. I know it seems weird, but there is a direct correlation between both, which is what I’ll be talking about today.
You hear people often talk about changing tire size and getting incorrect speed readouts. Is there any truth to this, or you shouldn’t worry about it at all? Let’s find out.
Before I explain the correlation and answer the big question, let me explain how things work. It will help you get a better understanding of the topic. Everyone knows what a speedometer is, so I’ll skip that definition and move on to explain how it works.
How do Speedometers work?
Speedometers are designed to show you the speed at which you’re driving, but how do they do that? It depends on the type of speedometer, and there are two types of those.
The more common and often referred to as the simpler system is a speedometer driven by a cable. In most cases, the cable is connected to the transmission and is driven from a small gear. The speed of the gear is determined by the rotation of the wheels, meaning that the more rotations the tires make, the more the gear will turn and show a higher speed. In some rare cases, come car manufacturers have the cable connected to the wheel. Overall, the working principle is the same; the difference is what spins the cable.
Newer cars, especially the ones with digital clusters, rely on digital speedometers. In this case, the speedometer relies on a sensor in the transmission or the wheels. As the car is moving, the sensor relies on impulses from the spinning assembly, regardless of which one it is, and based on the frequency of the impulses, it can show the speed. There is some math involved, and thanks to an algorithm, the car’s ECU can calculate the speed and show it on the dashboard.
The overall process may seem simple, but in reality, it’s quite complex. To be fair, it’s not something most people would be concerned with. The important aspect here is to know the basics so that I can continue with the guide.
Why Do Speedometers Not Show the Real Speed?
If you’ve ever driven your car using your phone as a navigation device, you may have noticed a speed difference between the speedometer and the phone. Let’s say the phone isn’t as precise, so you use specialized gear. Regardless of what you use, there will be a discrepancy, but why is that? There are several reasons why your speedometer will always be optimistic and show that you’re driving at higher speeds.
The most important one is that car manufacturers want to spare themselves from expensive lawsuits, and with good reason. If you’re driving on the highway doing 70 and the speedometer doesn’t overread, you risk getting a fine. With most cars, you’re getting a needle, so it’s not the most precise readout in the world. You may think you’re doing 70, but you’re doing 71, so you can get fined. This means that you also have the opportunity to sue the car manufacturer because the speedometer showed 70. It’s not as simple as this, but manufacturers want to avoid a potential scenario. This is why your speedometer will show more than what you’re actually driving. You’re looking at small differences, such as the speedometer showing 70 mph, but in reality, you’re driving a few mph less.
Another reason why speedometers show more is regular wear and tear on your car’s components. I’m going to give you a spoiler here, but as the tires wear down, the readout will vary a bit. To ensure that things remain within those tolerances’ boundaries, the speedometer will show an incorrect speed. Even when the tires or some of the other components wear down, it won’t result in the speedometer giving you an incorect reading.
As far as tolerances are concerned, there is another reason why speedometers show more. Since there is a correlation between tire size and speedometer readout (I’ll explain why in the following section), car manufacturers need to take size differences into consideration. Different models with the same size will vary a bit in diameter. Combine this with the tread depth, which, as it reduces, so does the diameter of the wheel, and the speedometer will start to show different readouts.
Why does Tire Size Affect Speedometer Readings?
To give you a better understanding of how different tire sizes can affect speedometer readings, first, you’ll need to understand how things work. As I mentioned before, the speedometer relies on predefined settings to ensure that you’re getting the right speed readout. Regardless of whether we’re talking about mechanical or digital speedometers, they are tuned from the factory to work with a certain tire size.
This means that the car manufacturer has taken into consideration the diameter of the tire along with the wheel. Based on that, the circumference is calculated, which determines the readout. At certain speeds, wheels with certain diameters make a certain number of revolutions per minute or mile. Without diving into complex mathematical calculations, you should know that, in most cases, once you change the tire size, the speedometer will continue giving you readouts based on the old size.
Depending on which tire size you choose, the discrepancy in the readouts can go either way. Your speedometer may give you a higher or lower reading, which is why there are a few ways in which you can remedy this issue.
Should you change your tire size?
Does this mean that you shouldn’t change your tire size ever? No. Changing the tire size is a common modification with most people, and there are ways to continue getting the correct readout from your speedometer. After all, I already did a guide on changing your tire size, so it’s not something you shouldn’t ever do.
If you’re one of those people who want to change your tire size, you have two options. In my opinion, the first one is the simpler one, and I believe it’s the one that most people would consider. I’m talking about upgrading your tire size while maintaining the wheel and tire diameter so they’re as close to the original as possible. As a result, the speedometer will continue working with the same circumference, meaning that the revolutions per minute or mile will be more or less similar. This means this speedometer will continue giving you the same readout as before.
This is achievable by making changes primarily in terms of the tire sidewall. Here’s an example. Let’s say you have a tire with 225/45R17 size, and you want to upgrade the wheel size to 18-inch. This tire size has a total diameter of 25 inches, making 808 revolutions per mile. To keep the total diameter as close to the original as possible, once you go for a wheel that’s one inch bigger, you’ll need to reduce the profile. The closest you can get while maintaining the same tire width is to have a 225/40R18 tire size. As you can see, you’re going from 45 sidewall to 40. This means that the total diameter of the tire will be 25.1 inches, which is about a 0.4% increase, resulting in 804 revolutions per mile.
Now you’re thinking: shouldn’t everything be the same? Ideally, yes, but in reality, there is some room for error, which is why I mentioned the tolerances before. In the example above, the difference in readout will be far less than 1 mph, so you’ll hardly notice any difference.
The second option you have is to change the tire and wheel size and, with that, change the total diameter. This will definitely result in an incorrect readout on your speedometer, so the solution here is to do a bit of recalibration. The process depends on the type of car. If you have an older car with a cable-operated speedometer, you’ll need to upgrade or tweak that system, which in most cases means that you’ll need to change the speedometer gear in the transmission. For modern cars that rely on sensors, you’ll need to go through the OBD port and make the modifications through software.
I wouldn’t call these processes overly complicated, but you wouldn’t want to make any mistakes. Can you DIY this process? Of course, as long as you know what you’re doing, have all the tools and know the process. If not, I recommend taking your car to a shop and having the experts take care of that.
Conclusion
Whenever you make some modifications to your car, it’s essential to ensure that it continues operating as optimally as possible. You may think that upgrading your tire size is benign and you won’t have anything to worry about, but the reality is different.
With the fitment aside, where your main concern is rubbing, there are other aspects that you should consider, like the speedometer. There is a direct correlation between the tire size and the readout, meaning that ideally, you’d want to have the total diameter of the upgraded tires remain as close to the original as possible.
If that’s not an option or you have something else in mind, then upgrading the tire size won’t be an issue as long as you recalibrate the speedometer.