How to Adjust Speedometer for Bigger Tires

Table Of Contents


How to Adjust Speedometer for Bigger Tires

There are many ways we can upgrade our vehicles. Whether it’s about performance, or aesthetics, tires and wheels are some of the first things that come to mind. Bigger tires bring a lot, whether it’s about the look or performance, such as off-roading. Still, there is one thing to keep in mind – tires that feature different sizes compared to those that originally came with the vehicle can alter the car’s operations in many ways. The most common thing i that it affects the speedometer. This consequently leads to inaccurate speed readout, which can be an issue when it comes to overspeeding and many other things, especially when it comes to safety. In other words, accurate readings are essential for maintaining safety and legal compliance. Therefore, the first thing to do when you get bigger tires is to adjust the speedometer. And, the good thing is that you don’t need a mechanic or any other professional to do that. You can do it yourself and save a lot of money, and we are here to explain to you how.

Speedometer: How Does It Work?

Before we start, a few words about the speedometer itself and the way it works. Of course, we are talking about the part of the vehicle that is responsible for displaying information about the speed. Throughout history, the speedometer evolved and became way more accurate but the base principle of working is still there, with mechanical and electrical components. Modern vehicles rely on electromagnetism and its base rules. There is a magnet that rotates, placed in a small metal cup that is connected to the transmission or driveshaft. So, the rotation of the driveshaft is what the speedometer reads out, actually.

Another important part is a metal disc or drum. We call it an eddy current disc, which is stationary and induces current as the magnet passes by it. Basically, that magnetic field that interacts with the eddy current in the disc is what forces the disc to rotate. Then, there is a needle, to which the rotation is transferred.  There are all kinds of gears of mechanical linkages and the need deflects according to the speed of the vehicles. Of course, this is then transferred to the speedometer dial you have on your instrument cluster, where you get the readouts. In simple words, it’s all about converting the rotational motion of the driveshaft into the rotational motion of the speedometer needle, all based on the rules of electromagnetism.


Importance of Speedometer Calibration

So, what’s the deal with larger tires? Well, when you install larger tires, you get a larger circumference of your wheels, which leads to inaccurate readings. The readings vary depending on the difference in circumference compared to the original tires. So, with larger tires, inaccuracy gets bigger in the way that, for example, if you install 5% larger tires, the speedometer displays a speed that’s around 5% lower.

So, what you need to do is to calibrate the speedometer and ensure accurate readings, because that’s the way to maintain safe driving conditions. If readings are not good, many issues can occur. Of course, the first thing that comes to mind is that you can exceed the speed limit, without being aware, which is a great safety risk. Moreover, it is also a shortcut to receiving a speeding ticket. There, there is the matter of fuel efficiency and its calculations, because the board computer will give approximations based on the size of the original tires. If not calibrated, the speedometer can affect other systems in a vehicle as well. Just think of things like cruise control and traction control. All in all, without calibration, you cannot count on safe and efficient driving conditions. 

Importance of Speedometer Calibration


How to Adjust Speedometer for Bigger Tires: Important Steps

Calculate the Difference

Tires come in different proportions but when we talk about the size, we usually think of the diameter. That’s what makes the difference when it comes to the speedometer and its readings. So, the way to start adjustment is to calculate the difference in diameter between the new and old ones. That’s super-easy because the actual size of a tire can be found on its sidewall. Once you get the diameters of the new and old ones, just subtract the diameter of the original from the new tires, and you will get the difference.

After you’ve done that, you have a relevant piece of information, on which you can base your adjustments. Logically, with the larger difference, larger adjustment is needed and there are a few common methods of adjustments. For example, there is a so-called speedometer or programmer, but it’s possible to do all this manually. Finally, you can give a professional mechanic to do this, but that will cost you. 

  • To calculate the difference, subtract the diameter of the original tires from the diameter of the new tires.
  • The larger the difference in tire size, the more significant the adjustment needed for the speedometer.
  • Common methods for adjustment include using a speedometer calibrator, consulting a professional mechanic, or manually adjusting settings.

Determine the Gear Ratio

From this point the determination of the heart ratio is. If you do not know exactly what is a gear ratio is a relationship between the number of teeth/splines on two interlocking gears inside of your transmission. And most importantly, how many times does your driveshaft need to spin to make your tires spin once around? Find out if is it difficult or possible. The easiest way is to ask the manufacturer or read your owner’s manual. If you prefer you can make a real check of the differential. Now you have two things to collaborate on – the difference in tire diameter and the gear ratio. All speedometer readouts are designed based on the idea of how quickly your tires rotate in relation to the driveshaft. So, this makes sense as long as your speedometer adjustments take into account both the size and number of cogs on your transmission.

  • The gear ratio represents the relationship between the number of teeth on gears in the transmission system.
  • You can find the gear ratio in the owner's manual, by checking with the manufacturer, or by inspecting the vehicle's differential.
  • The gear ratio affects how fast the tires rotate relative to the driveshaft, directly impacting speedometer readings.
  • Adjustments to the speedometer should consider both the larger tire size and the specific gear ratio of the vehicle.

Use a Speedometer Calibrator

The easiest way to recalibrate the speedometer is probably the use of a speedometer calibrator. This is a device that modifies the signal sent to the instrument. In this way, it modifies the readouts and, since it is just perfect, the device takes no time to learn how to use. That is right; you simply plug the calibrator in and start using. If you wish to recalculate the change in the tire size and the gear ratio, do not worry! You just introduce the numbers in the device software.

Convenience and easiness of use are just some of the benefits. Using such a calibrator is also usually the most precise way of adjusting, especially when compared to manual adjustments. Then, there are many products on the market that offer additional features, such as odometer correction, diagnostics, and similar things. 

  • They are easy to install and operate, often featuring plug-and-play functionality.
  • Calibrators allow input of new tire size and gear ratio for automatic adjustments.
  • They ensure accurate and reliable speedometer readings, even with larger tires.
  • Many calibrators offer additional features like odometer correction and diagnostic capabilities.

Consult a Professional

Consulting a professional is always a good move because they have the knowledge and experience. They can help you with adjusting speedometer settings, but they can also give you valuable info about many other things in the adjustment process. They can help you select the right method, they can introduce you to potential issues that may occur, etc.

That’s not all, because professionals also have a lot of tools, including specialized equipment and diagnostics that can streamline the adjustment and ensure the best possible results. With such tools, professionals can perform comprehensive checks and verify accuracy, do fine-tuning, and achieve perfect calibration.

  • Professional mechanics possess the knowledge and experience to adjust speedometer settings accurately.
  • They can offer guidance on selecting the appropriate adjustment method and identify any potential issues.
  • Mechanics have access to specialized equipment and diagnostic tools for streamlined adjustment processes.
  • They perform comprehensive checks to verify speedometer accuracy and make fine-tuning adjustments as needed.
  • Consulting a professional ensures precise calibration and peace of mind.

Electronic Adjustments

There are some vehicles with onboard diagnostics that feature among other settings the speedometer. Through an OBD scanner compatible with your ECU, it is possible to access and adjust the speedometer settings via the different software provided. This method does not require any manual adjustment or additional hardware; as such, it will be easy enough. 

With electronic adjustments, you can feed the new tire size, and at times the gear ratio, into the ECU directly, and it will adjust your speed readings. This method is easy, especially when you deal with an advanced electronic system in your vehicle. The beautiful part is that no hardware should cur/pages/tools be involved in this setting. This process can be easy to accomplish with the help of the OBD scanner and the software by just following the prompts.

  • Electronic adjustments utilize onboard diagnostics (OBD) systems to modify speedometer settings.
  • Compatible OBD scanners and software are required to access and modify settings directly from the vehicle's electronic control unit (ECU).
  • Inputs such as new tire size and gear ratio can be entered into the ECU for recalculating speedometer readings.
  • This method eliminates the need for manual adjustments or additional hardware.
  • Electronic adjustments are convenient, efficient, and suitable for vehicles with advanced electronic systems.

Electronic Adjustments


Manual Adjustment

Manual adjustment requires the driver of a vehicle to make physical modifications to the speedometer system to fit larger tires. This is usually an underlying solution for older vehicles or those that lack the systems for electronic speedometer calibration. The adjustments of a manual system range from replacing the speedometer and odometer gears; modifying the speedometer cable; or at worst, resetting the speedometer mechanism. Manual adjustment is relatively challenging and labor-intensive, making it less common today. For this reason, a manual system would call for mechanical experience or professional calibration. 

A more common manual adjustment method consists of changing a speedometer gear located in the transmission or the axle to the new tire size’s ratio. Such gears possess varying tooth counts, influencing the speedometer’s output. Thus, selecting an appropriate tooth count on a gear for larger tires will recalibrate the speedometer manually and ensure precise readings. Moreover, changing the speedometer cable or the gearing transmission will adjust the calibration. Although manual adjustment methods take more time and effort, they are viable for vehicles where electronic recalibration is not available or clients prefer hands-on customizability.

  • Manual adjustment involves physically modifying speedometer components.
  • It may be necessary for older vehicles or those without electronic calibration capabilities.
  • Methods include replacing gears, modifying the speedometer cable, or recalibrating the entire mechanism.
  • Changing the speedometer gear in the transmission or axle can recalibrate the speedometer for larger tires.
  • Manual adjustments may require mechanical expertise or professional assistance.

Test Drive and Calibration

Of course, once you’re done with the speedometer adjustment, it’s time to take your car for a test drive. In that way, you will check if the speedometer offers accurate readings now. Try your vehicle at various speeds, on different types of roads, both local streets and highways. Make sure to compare the speedometer readouts with GPS devices, because it’s the best way to verify accuracy and identify any issues that may require additional calibration.

If necessary, you can then do a fine-tuning. If there are some minor discrepancies, you can make additional adjustments with one of the methods we already mentioned in this article. Rechecking for additional adjustments is a normal thing and if a struggle occurs, consult a professional.

  • Conduct a test drive at various speeds on different types of roads.
  • Compare speedometer readings to actual speeds indicated by roadside speed signs or GPS devices.
  • Verify the accuracy of the speedometer adjustments during the test drive.
  • Fine-tune speedometer calibration if discrepancies are observed.
  • Additional adjustments may be necessary using the chosen calibration method.
  • Iteratively test and calibrate the speedometer to ensure reliable and accurate readings.

Summary

All in all, once you install bigger tires, the first thing to do is to calibrate the speedometer, to ensure accurate readings and maintain safe driving conditions. The variations in readouts are caused by the difference in the tire diameter and they can lead to various safety issues, legal implications, and inefficiency of other in-car systems. With proper calibration, you are avoiding all these risks, which is crucial for safe and responsible driving.