How to Recalibrate a Speedometer After a Tire Size Increase

Cars are so prevalent nowadays that people forget how complex they are. Modern vehicles not only give you the means of covering large distances quickly, but they also do it comfortably and safely. Not to mention, cars can give you all sorts of information about how they function and come with advanced hi-tech interiors with numerous apps you can use to improve the traveling experience.

However, the single most important information you need in your car is the speedometer. This device ensures that you always know how fast you are driving, which has numerous implications. For instance, it can save you a ticket, but it can also tell you if you are going too fast for the road conditions you are driving on.

However, the speedometer's accuracy is closely related to the tire size. So, whenever you put larger or smaller tires in diameter, for example, if you go for huge flotation tires on your SUV or low-profile tires for your sports car, it will mess with the reading on the speedometer.

So, today let’s find out how to recalibrate a speedometer after a tire size increase.

Why Speedometers Never Show the True Speed

You probably don't know this, but getting an accurate reading from the speedometer isn't easy. Even a new car that you just bought from the dealer lot won't tell you an accurate reading of the speed. Instead, it will usually give you a higher reading, which is erring on the cautious side. This phenomenon is sometimes referred to as "speedometer optimism."

That way, the automaker ensures it is legally covered, i.e., if the speedometer reads 70 mph, you are actually traveling at 67 mph, meaning you won't get a ticket. If the automaker designed the speedometer to show a lower speed, you might see 70 mph, but the actual velocity would be 75 mph, resulting in a ticket. Naturally, automakers don't want to get into such trouble.

Furthermore, speedometers can't always show the exact speed due to manufacturing tolerances. These tolerances are inevitable small variations that can occur in the components during their manufacturing process. Manufacturers often design their speedometers to err on the side of higher-than-actual speeds to ensure that even with these tolerances, the speedometer does not underreport the speed.

One part of these tolerances is the wheels and tires. Namely, even a tire with the same size, say 205/55R16, will have a slightly different diameter depending on the make and model due to the tread depth. Moreover, tires wear over time, which decreases their diameter, which inadvertently messes with the readings on the speedometer.

Not to mention, other parts in the drivetrain, like, for example, gears, also wear down, which can also affect the accuracy of the speedometer. Thus, erring on the side of caution, i.e., showing a higher reading on the speedometer, is always preferred.

Understanding the Basics of Speedometer Calibration

Before we dive into recalibration, let's cover the basics and understand why your speedometer needs calibration after a tire size increase or decrease.

1. What is a Speedometer?

The speedometer is a device built into the dashboard that shows the speed at which the vehicle moves. It should always be easily visible and readable, even during the night. For most of automotive history, the speedometer was analog (mechanical), with a needle moving across printed speed readings. Modern cars, though, increasingly use digital readings, which are more accurate and readable. The speedometer shows the speed in kilometers per hour (km/h) or miles per hour (mph), depending on the market, or both.

The mechanical speedometer used in older vehicles (and even some new cars) uses a flexible, spinning cable. This cable is driven by a gear in the transmission, which is turned by the rotation of the wheels. As the vehicle's speed increases, the cable spins faster. Then, it transmits the force through a series of gears to the speedometer, which shows the reading with a needle.

An electronic speedometer

Meanwhile, digital (electronic) speedometers use speed sensors located on the transmission or the wheels. The sensor gets a series of electronic pulses, which frequency increases with the increase in speed. The data then goes to the ECU, i.e., the vehicle's onboard computer, which processes it and send it to the speedometer.

2. The Impact of Tire Size on Speedometer Accuracy

The diameter of the tires plays a crucial role in the accuracy of the speedometer. Namely, as the diameter increases, tires cover more ground with one rotation. Because the speedometer bases its calculations on the standard number of rotations for the original-sized tires, it will under-report your speed with larger tires.

Meanwhile, smaller tires cover less distance for one rotation. This leads to the speedometer over-reporting the speed because it expects each rotation to cover longer distances. As such, this is a smaller issue since it will keep you away from tickets, but still important to tackle if you want to improve the driving experience.

The impact of tire diameter on your speedometer readings can, fortunately, be calculated. Namely, if you put tires that are 10% larger than your stock tires, the speedometer will read approximately 10% lower than the actual speed.

With that said, other factors are also into play here, namely the tire load and pressure. Underinflated tires, for instance, will cover slightly smaller distances, resulting in a higher reading than usual, while overinflated tires could under-report the speed readings.

Assessing the Need for Speedometer Recalibration

Before recalibration after a tire size increase, it is recommended to assess the need for a speedometer adjustment. Check my following tips on how to do that.

1. Determining the Difference in Tire Sizes

First of all, it is crucial that you measure the diameter of your new tires precisely. By knowing the difference in tire sizes, you can estimate how much the speedometer reading might be affected in percentage points. I recommend measuring the diameter with fully-inflated tires that are already installed on your vehicle.

2. Testing Speedometer Accuracy

In the past, testing the speedometer accuracy was only available to professional mechanics. Now, though, you can pull the smartphone from your pocket, launch any navigation or GPS app, and see the speed reading.

The GPS sensor on your smartphone uses data from the Global Positioning System and has nothing to do with the tire size. Thus, it always gives the true speed reading. With that said, it's important to note that smartphone GPS sensors aren't that accurate. They will still show a more accurate reading than the speedometer after a tire increase but won't give you the exact speed.

If you want to be more precise, you can use a performance meter. These devices also use GPS sensors, but they are of a higher quality and much more accurate, down to 0.1 mph or km/h. As a bonus, you can also measure your 0-60 mph or 0-100 km/h time on these meters or completely replace the speedometer in your car.

Methods for Recalibrating a Speedometer

Recalibrating a speedometer isn't a straightforward job and depends on your DIY comfort level. If you are ready to tackle the issue yourself, be prepared to spend some time working on your car, as it could take several hours on analog speedometers or up to an hour on digital speedometers.

1. Manual Speedometer Recalibration

Changing one of the gears to a new one with a different number of teeth would adjust the speedometer reading

You can recalibrate a manual speedometer by either changing its gears or adjusting the cables.

The former is more straightforward for beginners, as it doesn't require a lot of know-how and experience. Basically, you will need to change one of the gears to a new one with a different number of teeth, which would adjust the speedometer reading.

However, I strongly suggest talking to a professional about the correlation between the tire size increase and the size of the gear. You can find a lot of information online, particularly on off-road forums, where owners have already installed large flotation tires on popular SUVs like the Jeep Wrangler and Toyota 4Runner, and then replaced particular gears to recalibrate the speedometer.

Adjusting the cable isn't as straightforward, though, and might take a lot of time. Basically, you will be playing with the length of the cable to fine-tune the speed readings, which means you will need to drive your vehicle several times, re-check the reading, and re-adjust the cable.

2. Digital Speedometer Calibration

Those that have a vehicle with a digital speedometer are in luck because recalibrating it should be a breeze. You will find an OBDII speedometer calibration device for this, where you will just enter the diameter of the new tires, and the device will basically reprogram the speedometer.

That said, there are various speedometer calibration devices on the market, each for different vehicle makes and models. Thus, it's paramount to choose one that works with your particular vehicle.

You can also connect your laptop to the OBDII port in your car and use tuning software to recalibrate the speedometer after a tire size increase. This is a more advanced approach, and I would only recommend it to people that know how a thing or two about vehicle tuning.

3. Professional Speedometer Recalibration Services

If you're not comfortable with recalibrating the speedometer, consider taking your vehicle to a professional. Mechanics or specialized service shops have the necessary equipment and expertise to recalibrate your speedometer accurately. They will also be able to test the speedometer after recalibration to confirm it's reading correctly.

Common Pitfalls and Precautions

It might seem like a mundane task, but you should approach installing larger tires and, subsequently, recalibrating the speedometer very cautiously.

1. Legal Implications of an Incorrect Speedometer

The main reason you would want to make sure your speedometer gives you an accurate, or a slightly higher reading, is to avoid breaking speed limits unknowingly. If your speedometer is under-reporting your speed because of a tire size increase and you rely on it to stay within speed limits, you could be at risk of receiving speeding tickets.

But even more importantly, speeding can significantly increase the risk of accidents happening. Even a 10% higher speed could significantly increase the braking distances of your vehicle, sometimes more than 5 ft (1.5 meters). Moreover, your vehicle will be less stable at higher speeds, and you will have less tire traction to play with.

Not to mention, the higher the speed at the impact, the more severe a crash tends to be. As speed increases, the amount of kinetic energy that needs to be absorbed by the impacts of a collision increases exponentially, which can lead to more severe injuries and damage.

According to the Transport Research Laboratory, an increase in speed of just 1 mph increases the chances of accidents happening by approximately 5%. This means that if the speedometer underreports the speed by 10%, the accident rate is increased by 50%. Speed is no joke, guys!

2. Ensuring Proper Installation

Installing new tires doesn't only require speedometer recalibration – you will need to ensure they are properly mounted, balanced, and inflated.

In cases where the tire size increase is very high, you might also need to lift the suspension to minimize fender rubbing. Don't forget that lifting the suspension also requires a wheel alignment.

3. Effect on Your Vehicle's Performance

Larger tires don't only meddle with the readings on your speedometer but also with how your vehicle handles. Notably, larger diameter tires can seriously impact the handling of your vehicle and make it less stable. They will also reduce the lateral grip, braking distances, and acceleration in a negative way.

4. Regularly Checking Your Speedometer

Even after you've recalibrated your speedometer, it's important to check its accuracy regularly, particularly if you often change between sets of tires with different sizes, like winter tires and summer tires.

Conclusion

So, what do you think – are you ready to recalibrate the speedometer yourself? If it was up to me, I wouldn't care about getting my hands messy, despite my experience in the shop. Professional mechanics can do it for me in just 30 minutes while I will be drinking a nice warm coffee.

But you might still need to do it yourself if you own an older SUV/truck with a mechanical speedometer. In that case, use all the knowledge from the internet you can get, and ensure you've done a great job with the recalibration using a GPS tool. Good luck!