Speedometer Calibration: Everything You Need To Know
Knowing your driving speed is essential when you are on the road, but you should make sure that your speedometer is properly calibrated for accuracy.
When do you need to calibrate your speedometer?
Your vehicle’s speedometer will need to be calibrated after you upsize or downsize your tires (or when they are worn out or under-inflated). You can calibrate your speedometer by using your vehicle’s speedometer calibration button or by taking it to a professional auto shop.
After extensively researching automotive troubleshooting forums, I have been able to gather enough information to determine how to calibrate a speedometer and why keeping your speedometer accurate is important.
My research has indicated that the most common reason that speedometers need to be re-calibrated is due to changes in tire size or condition.
What Does Speedometer Calibration Do?
Whenever you drive, your vehicle displays the speed that your car is currently moving at on the speedometer. This allows you to maintain speed limits and drive your car with accuracy.
Aside from your brakes, understanding your driving speed is one of the most important aspects of operating a vehicle. However, to confidently know what speed you are driving at, your speedometer needs to be calibrated.
Speedometer calibration is the process of measuring your car’s speedometer for accuracy to ensure that you are getting a proper reading when on the road. If you feel that your speedometer may be off, then you should absolutely get it calibrated.
There are a couple of different ways that you can calibrate a speedometer – depending on how accurate you want the process to be.
- DIY speedometer calibration
- Professional speedometer calibration
These days, just about every car is equipped with a feature that allows you to calibrate a speedometer yourself. This is particularly useful given that speedometer calibrations should happen as soon as there is any indication that the reading may be off.
On the other hand, relying on professional help for calibrating your speedometer is the most guaranteed approach for ensuring that the calibration was done right, which is why a lot of people like to take their vehicles to mechanics for the job.
Why is Speedometer Calibration Important?
The reason why speedometer calibration is important is that you want to know how fast you are driving your car. Speedometers can lose accuracy for several different reasons and if you are driving at an incorrect speed, it can result in a number of different issues.
First and foremost, depending on how uncalibrated your speedometer is, you could potentially be driving faster than you think you are.
While this is a major safety hazard and that alone is enough reason to keep your speedometer accurate – speeding is also illegal! By having a calibrated speedometer, we are able to observe the speed of our cars while driving so that we can obey traffic laws.
Alternatively, your speedometer may read that you are driving faster than you actually are. This can result in unnecessarily slow speeds, which means that you are going to be less efficient on the road and you may potentially contribute to traffic congestion.
Are Speedometers Accurate?
Ironically, speedometers are rarely, if ever, truly accurate. When a car manufacturer designs a vehicle, they intentionally make the speedometer slightly inaccurate.
This is often shocking for a lot of drivers to discover. Every time that you get behind the wheel of a car, whether it is brand new or old, the speed that you are driving is off by at least 5% or more.
While it may seem counterintuitive to design a vehicle with an inaccurate speedometer, car manufacturers do it intentionally. The main reason behind this is so that the speedometer can better adapt to the speed that your car is traveling at after the tire changes form due to wear and tear. More on this next.
A general rule of thumb is that your car’s speed is within 5% of what your vehicle is actually driving at, but some estimates can be as high as 10%. Ultimately, the car manufacturer that designed your vehicle will decide how off your speedometer really is.
How Do Speedometers Lose Accuracy?
When dealing with speedometer calibrations, it is important to understand why speedometers become less accurate in the first place.
As I just covered, your car’s speedometer was never 100% accurate in the first place. So, you can expect that even the most sophisticated and thorough calibration is not going to be completely spot on.
However, having a speedometer that is within the ballpark range of your driving speed is mandatory for a number of reasons. That is why you should re-calibrate whenever you encounter a situation that changes the accuracy of your speedometer.
Downsizing Tires
The most common reason for a speedometer to become less accurate is when you change your vehicle’s tires. If you downsize to a smaller tire, then you can expect your speedometer to read that you are driving faster than you actually are.
The smaller tire has a smaller circumference resulting in your speedometer giving you a faster reading. A small tire installation will particularly lower the accuracy of your actual speed given that your vehicle’s speedometer should give a faster reading based on the manufacturer’s design.
Upsizing Tires
On the other hand, installing a bigger tire onto your vehicle will have the opposite effect. Instead of driving faster than your actual speed, you will think you are driving slower.
Given that the circumference of a large tire is bigger, the reading that you get from your speedometer will be that you are driving slower. Unlike downsizing, upsizing might actually bring you closer to your legitimate speed – since speedometers are designed to read faster.
Calibrating your speedometer is particularly important after you upsize your tires given that you do not want to be caught unnecessarily speeding, as this could easily result in a speeding ticket, as well as increased safety risks.
Under-inflated Tires
Keeping your tires inflated to their correct PSI is going to enhance your vehicle’s performance first and foremost, but it will also affect how accurate your speedometer is. If you are driving with under-inflated tires, you can expect your speedometer to be off.
Under-inflated tires will cause you to think that you are driving faster than you actually are. How off the reading is will ultimately be determined based on how deflated your tires are, but you can expect an error of roughly 5% once the under-inflated tire has sunk 1 inch closer to the ground.
Worn Tires
If your tires are due for a replacement, you might notice yourself getting from point A to point B a bit slower than usual. Much like with downsizing your tires, your vehicle’s tires are going to get smaller as they get worn out.
This will ultimately cause your speedometer to give you an inaccurate reading and you will think you are driving faster than you are. So, if you want to keep your speed accurate, make sure to change your tires on time.
Who Does Speedometer Calibration?
The good news is that calibrating a speedometer is not very difficult. Virtually every car that you find on the road is equipped with a feature that allows you to re-calibrate the speedometer yourself so that you can keep your vehicle’s speed as accurate as possible.
The DIY route is certainly the most practical, straightforward, and affordable approach you can take. So long as you follow the instructions correctly, your vehicle’s speedometer calibration should be pretty spot on.
However, a lot of drivers prefer to get a professional speedometer calibration.
Doing so does come with certain perks – most importantly, accuracy! A professional mechanic that is experienced with speedometer calibrations is going to deliver the best results, which is particularly important for certain people who drive for work.
Now, as I have already covered, speedometers are never completely accurate, as car manufacturers intentionally design vehicles this way. So, even with the best calibration imaginable, you should still expect your speedometer to be off by at least 5%. With that said, let’s dive into how you can calibrate your car’s speedometer.
How to Calibrate a Speedometer DIY
If your car is equipped with a feature that allows you to calibrate the speedometer DIY, you can take care of this on your own without needing to seek out professional help.
You should be able to find a calibration button near your vehicle’s speedometer. Once you have found the speedometer calibration button, follow these steps:
- Press & hold the calibration button
- Turn on your car
- Release the calibration button
- Press that calibration button (again)
- Test drive the car
- Drive the distance needed (as displayed)
- Press the calibration button upon completion
After you drive the vehicle and press the calibration button, your speedometer should be calibrated and you will get a fairly accurate reading.
If you are unsure of whether your vehicle has this function, then you should check your owner’s manual, as this will give you more detailed information about your vehicle’s specific make and model.
How to Calibrate a Speedometer Professionally
A DIY speedometer calibration should be fine in most cases, but you may feel inclined to get it done professionally. Although the procedure is not free, it is the best way to ensure that the job was done right and that your speedometer is calibrated optimally.
The best way to get your speedometer professionally calibrated is to go to an auto repair shop, as they will have the tools necessary. A professional shop will hook up your vehicle to a dynamometer and they will test your speedometer for accuracy, which is not something that you will be able to do at home.
How Much Does Speedometer Calibration Cost?
The price to get your speedometer professionally calibrated can vary depending on the shop that you go to. However, it’s normally not very expensive, as it’s a simple procedure – with most shops charging anywhere from $50 to $100.
If you want to save yourself money on your speedometer calibration, the best way to do this is to take the DIY route and use your vehicle’s calibration button, as this is free!
How Long Does a Speedometer Calibration Take?
Calibrating your vehicle’s speedometer does not take long. So long as you are only getting your speedometer calibrated with no other service repairs, then an auto-shop should be able to do it in 20 to 30 min.
However, doing a DIY calibration can often take even less time – with 10 to 15 min being the average for most drivers.
Will Speedometer Calibration Void Warranty?
Drivers are often skeptical about calibrating their speedometers (especially on new cars), as they fear that doing so will result in complications with their warranty. However, calibrating your car’s speedometer should never void the warranty of the vehicle.
Although some modifications can void the warranty of a vehicle, a speedometer calibration is under no circumstance a legitimate mod and it falls under standard maintenance and upkeep.
Are Speedometer Calibrations Legally Required?
A lot of drivers that have neglected calibrations often wonder whether keeping their speedometers accurate is even a legal requirement. The honest answer is – it depends.
You need to be driving at a legal speed, which your speedometer certainly helps you achieve. What it really comes down to is how inaccurate your speedometer is and whether you are breaking any laws by driving with it uncalibrated.
Federal law dictates that drivers may not drive a car with a speedometer that reads more (or less) than 10% of the actual speed. So technically, if your speedometer’s calibration is not more off than 10%, you aren’t breaking any laws.
Key Takeaways
- Your vehicle’s speedometer will need to be calibrated after you upsize or downsize your tires (or when they are worn out or under-inflated).
- You can calibrate your speedometer by using your vehicle’s speedometer calibration button or by taking it to a professional auto shop.
- All speedometers are designed to be inaccurate by about 5% – with estimates usually being higher than the actual speed of the vehicle.
- Speedometer calibrations usually cost around $50 to $100 at a professional auto shop – or you can do it for free by using your vehicle’s speedometer calibration button.