Everything You Need To Know About Speedometer Calibration

Speedometer Calibration: The What, Why & Where

You must be able to tell how fast you’re going when you drive. An inaccurate speedometer can put you in great danger and you must get the issue fixed as soon as you notice it. But you might be wondering where you can get speedometer calibration and how it works.

Auto speedometers aren’t particularly complex devices, but they are very sensitive. Even meters on brand-new cars that just rolled out of the factory can give you inaccurate readings. Read on to learn everything you need to know about speedometer calibration in Virginia, including where to get speed calibration service.

How Does a Car Speedometer Work?

A speedometer — or speedo — serves a simple but vitally important purpose. Whether it uses an old-fashioned needle dial or has a cutting-edge digital display, the speedometer tells you how fast your car is moving. But how exactly does the speedometer know your car’s speed?

The answer might surprise you. It’s because of magnets!

All speedometers use magnetism to measure the number of revolutions your vehicle’s driveshaft does per mile. This measurement is transferred to your speedo display. In general, cars have one of two types of speedometers — mechanical or electronic.

Mechanical Speedometers

Mechanical (or eddy-current) speedos are connected to your car’s driveshaft with a long and flexible cable. The connection point is “downstream” of the gearbox. This means changing gears doesn’t affect the measurement and your speedo reading won’t suddenly change when shifting.

As your engine runs, the driveshaft rotates and turns the speedo cable around as well. The cable in turn spins a magnet housed inside a speed cup.

The magnetic currents will cause the speed cup to turn in the opposite direction of the magnet. A hairspring between the cup and the display dial tightens as the cup turns, limiting its range of movement. As the cup turns, it moves the needle on your speedo dial, indicating your car’s speed.

Mechanical speedos have remained virtually unchanged for decades, and they’re still common in modern cars.

Electronic Speedometers

Like many other car components, speedometers are increasingly becoming electronic. Electronic speedos also measure driveshaft revolutions, but they use a different operating principle.

An electronic speedo relies on a magnet connected to the driveshaft. As the driveshaft rotates, this magnet passes another one, triggering a sensor. The sensor sends the number of revolutions to a circuit, which amplifies the signal and sends it to the speedo display.

Electronic speedometers often have digital displays that show your speed in numbers. But they can also be connected to an electric motor that moves a needle, like on a mechanical speedo.

Electronic speedometers are in general more durable and accurate than mechanical ones. They are also more expensive, though, which is why even modern cars sometimes use mechanical meters.

What is Speedometer Calibration?

Just as any other car component, speedos will eventually experience wear and tear. This can cause them to become inaccurate and not display your car’s true speed. Even new cars can have their speedo readings off by 1-3 mph.

To correct the issue, you need speedometer testing. During this test, a technician will measure your car’s true speed and compare this reading to the number displayed on your speedometer.

Technicians use a device called a dynamometer to determine your vehicle’s true speed by measuring tire rotations. They will increase the speed gradually over 10 mph increments. As they work, the technicians will jot down the true speed and the speedometer reading on a calibration report.

If the technician notices that readings are significantly different, they will recommend speedometer calibration to you. They can adjust the speedo components so that your car’s display shows you an accurate speed.

What Can Cause Speedometer Inaccuracy?

Many things can throw your speedo off the whack and cause it to lie to you. Some of the most common causes of speedometer inaccuracy include:

  • Wear and Tear: Mechanical speedometers have plenty of moving parts that will wear down with age, resulting in inaccurate readings. The sensors of electronic speedometers can also experience malfunctions or get caked with dirt and dust.
  • Accidents and Damage: Even a minor car accident could damage or jolt speedo components enough to throw them off. Road debris flying around can also result in speedo damage.
  • Aftermarket Rims and Tires: Speedometers are calibrated to your car’s stock wheel size by default. If you increase or decrease the wheel size with new rims or tires, the speed at which the wheels rotate will change and introduce inaccuracy to your speedo.
  • Tire Wear and Pressure: Worn tires or having low tire pressure can also cause your wheels to rotate at an unusual speed, which messes with your speedometer.
  • Heavy Loads: A heavily loaded car will slow down the rate at which your wheels turn and can result in inaccurate speed readings. Fortunately, this issue usually fixes itself once you unload your vehicle.
  • Manufacturer Design: Many car manufacturers intentionally calibrate speedometers to show a slightly higher speed than the vehicle’s true speed. This is a little white lie that is intended to keep drivers safer by preventing speeding.

How Much Does Speedometer Calibration Cost?

Speedometer calibration is a relatively inexpensive and fast auto maintenance service. Most repair shops charge a flat fee for the service. The average price of speedo calibration in Virginia is approximately $100.

The famous saying states that time is money. A speedo test and calibration won’t cost you much time either. It usually takes less than an hour, depending on how busy your car repair shop is. 

Why Do I Need Speedometer Calibration?

When driving, you must have an accurate idea of how fast your car is traveling. The reason is very simple — not knowing your speed is incredibly dangerous.

Every additional mph makes controlling your car and responding to risky situations more difficult. When you travel at great speeds, you have less time to hit the breaks or steer your car before a potential collision. Higher speeds also narrow your peripheral vision and make it more difficult to process the things you can see.

Making strong, sudden steering movements at high speeds can cause your vehicle to spin out of control or even roll. If you’re traveling faster than what your speedometer shows, you may try to steer too forcefully. The results could be disastrous.

Northern Virginia also gets a lot of rain and snow is common during our cold winters. Driving in these adverse conditions is challenging enough as it is. An inaccurate speedometer makes it more difficult to adapt your speed on slippery, snowy, or icy roads.

Last but not least, a lying speedo puts you at risk of speeding even in perfect weather. Speeding in Virginia can earn you possibly hefty fines and demerit points on your license that remain for multiple years. Some speeding infractions, like going too fast in a school zone, could even get you charged with a crime.

Can Speedometer Calibration Help with Speeding Tickets?

If you do happen to get a speeding ticket, a speedometer calibration test could help you lower the fine. Let’s say a police officer pulled you over for speeding and gave you a fine. But you paid close attention to your speed and know that you shouldn’t have gone above the limit.

What gives? It could be that your speedometer has gotten so inaccurate that you broke the speed limit without even realizing it.

According to Virginia law, courts can consider speedometer calibration reports as valid evidence for lowering a fine. It could also lower your offense from reckless driving to a lighter speeding infraction. If you can prove that your speedometer showed a lower speed than you were actually traveling at, courts may prove lenient.

But not just any speedo calibration report will do. The document has to be signed by your technician and properly notarized. Without these authentications, the courts can’t accept the report as valid evidence.

If you receive a speeding ticket and suspect your speedo reading at the time wasn’t accurate, it may be worth getting a calibration check. Make sure the repair shop doing the check can issue you a notarized report. 

Also, ask the technicians to include identifying information on the report, such as your car’s year, make, model, license plate, and VIN number. These details can help you prove the tested car is the same one you drove at the time of the infraction.

Get Accurate Speedometer Calibration in Chantilly and Warrenton, VA

At Advanced Automotive, we have been providing drivers in Chantilly and Warrenton, VA, with speedometer calibration services for more than 35 years. We guarantee that your speedo will give you an accurate reading.

Our ASE-certified and highly educated technicians are familiar with all car makes and models. We know how to diagnose and calibrate both mechanical and electronic speedometers to flawless accuracy.

Advanced Automotive can also help you in traffic court. We’ll give you two notarized copies of every calibration report. If you get a speeding ticket and know you weren’t going too fast, come see us — our technicians can help you prove your speedo was lying.

You don’t need an appointment for speedometer calibration at Advanced Automotive. Simply drop by at our shops in Chantilly or Warrenton and we’ll check your meter!