How To Calibrate A Speedometer?

When to calibrate your car’s speedometer?

Knowing how to calibrate a speedometer is very important because it’s vital to know how fast you’re going at a given moment.

It doesn’t only allow you to operate your vehicle safely, but also help you avoid the chance of meeting law-enforcement officer for going over the speed limit.​

You know there is something wrong with your car’s speedometer when you get a speeding ticket while you’re sure that your speedometer is registering the speed limit or when you’re continually being honked at by other drivers for driving too slow while the speedometer is showing the speed limit.

If those things happen, it’s time for you to calibrate your speedometer. In this article, we’ll be showing you how to calibrate a speedometer on your own.

Why is your car’s speedometer off?

Your car’s speedometer can be wrong, in fact, it will almost always register a faster rate than your actual speed.

Variations in vehicle load, tire temperature, tire wear, tire diameter will cause speedometer errors. The calibration tolerance range is usually within 1 to 5%.​

Tires

Over-inflation, under-inflation or wearing-down of tread can cause changes over time in your tire’s circumference. These changes can influence the speedometer’s readout. Your speedometer has to allow for a margin of error in order to be effective

Or maybe you just have bigger tires installed; you will need to calculate your true MPH again. The reason is, to the number of your tire’s revolutions is what your speedometer is reading, not how fast your tires are spinning.

Bigger tires mean each revolution will be further, making your actual speed higher than the reading on the speedometer.​

The four percent rule

All American and most Japanese manufacturers subscribe to the SAE J1226 standard, in which, a plus-or-minus 4 percent is acceptable.

With this four percent rule, you’re potentially going faster than what your speedometer reads at low speeds. However, you’re driving at least 3 mph slower at 100 mph.

CFR-393.82 standard

At 50 mph, a maximum 5 mph plus-or-minus error is allowed by Federal standards in the US. For private passenger vehicles, the speedometer can never read bellow the actual speed and must not exceed 110% of it.

The European formula

UN ECE Regulation is required by the European Union, in which, no speedometer is allowed to read slower than the actual speed. On another note, speedometers can read up to 10% above the actual speed, plus 4–6km/hour, depending on the vehicle type.

With this European formula, a speedometer can be nearly 13 mph off at 100 mph at most.

How to test your speedometer?​

To test your car’s speedometer, you have various methods.​

  • Speed radars

Many highway patrols place speed radars on the side of the road. You can determine how far your speedometer is off by driving past one of them.

Notice and compare the reading on your speedometer and on the speed radar to calculate the difference.​

  • GPS device or Smartphone app

​Get an application for your smartphone or a GPS device that can measure distance over time. They can be extremely accurate; however, some of them won’t take elevation changes into account which will lead to errors.

  • Use a stopwatch or a wristwatch

​This method is used by the police to calibrate their speedometers. You will need a chronograph with a tachymeter function.

Just cruise down your car at a steady speed. Start timing right when you pass a mile marker and stop it once you have passed the other.

How to calibrate a speedometer?

Calibrating a speedometer is not as hard as it seems. Most of the times, some basic math, the right equations, and an accurate standard of distance or speed are all you need to calibrate your car’s speedometer.

Replace the old speedometer with a new one

Mechanical drive speedometers are the choice of many drivers. Nowadays, every new car is equipped with electronic speedometers, both digital and analog.

Electronic speedometers are becoming the main lines of most of the aftermarket instrument and original-equipment suppliers.​

​The information on your speedometer gear can be found in your auto manual. Once you’ve found it, call an auto parts store or the dealership to let them know your type of speedometer gear and how far off the reading is.

To replace the old speedometer with a new one, what you’re going to need are needle nose pliers, an 11 mm wrench, a cloth, and your new gear.

  • ​The first step is to remove the old speedometer, use the cloth to wipe the automatic transmission fluid off it.
  • Take off the retaining clip using the pliers. In the speedometer assembly, remove the gear.
  • Put the new gear into the speedometer assembly and install it.
  • Place the retaining clip back on.
  • Place and secure the speedometer assembly back to the tail shaft by tightening the bolt. Line the speedometer cable up with the gear.

​Once you’re done installing your new speedometer, test it out to see if it works correctly. If it doesn’t, return the gear to where you get in exchange for a correct one.

Recalibrate a speedometer

  • Mechanical speedometer

To change the speedometer reading, you have to change the ratio between the drive and the driven gear.

The speedometer reading will be sped up if the driven gear has fewer teeth and slowed down if the driven gear has more teeth. Vice versa, the reading will be slowed down if the drive gear has more teeth and sped up if the drive gear has fewer teeth.​

Locate the transmission by opening the car’s hood. From the transmission’s tail shaft, unscrew the speedometer cable. Unscrew the bolt that holds the gear housing cover by a crescent wrench.

Expose the two gears by removing the plate:

  • ​Driven gear attached to the speedometer cable.
  • Drive gear mounted inside of the transmission. Note down the number of teeth on the drive gear.

​To determine the diameter of the tire, multiply the number of the measurement from the center of the tire wheel to the top of it by two. To determine the number of revolutions per mile, divide 20,168 by the tire’s diameter.

Multiply the amount of drive gear teeth by the number of tire’s revolutions per mile. Multiply the car’s axle ratio by this value. To find the car’s axle ration, look it up in the supporting document that your car came with.

Divide the solution that you’ve found down by 1,001. What you will have is the number of driven gear teeth that you need.

Call the local speedometer service or the car’s manufacturer to buy the right driven gear that you need. To ensure you receive the right gear, provide the year, model, and make of your car.

The old driven gear is in the end of the speedometer cable. Remove it by pulling it out of the clips. Clip the new on into place. Replace and secure the gear housing cover with the bold removed in the beginning. Secure the speedometer cable back.

You can do the same procedure in case you change your tires into bigger ones.

  • Electronic speedometer

There are 3 ways to calibrate an electronic speedometer with LCD display and they’re all relatively simple.

  • ​Automatically calibrated by driving on a road with the exact 1-mile distance.
  • Use the input of the car’s and sensor’s pulse-per-mile.
  • Use a reference point to adjust.

​While turning on the engine, press and hold the button on the front of the speedometer. The display on the LCD will change as you hold the button. Scroll through the 3 methods, stop for about 2 seconds on each option.

If you change your tires’ size, follow these steps:

To recalibrate your speedometer, it’s needed to determine the distance of the test drive.

You can find this information in the supporting documentation or the owner’s manual that came with the car. The distance is also prescribed by the speedometer manufacturer.​

  • ​On the speedometer, press and hold the calibration.
  • Start the car and release the button that you’ve been holding.
  • Press it again and drive the prescribed distance.
  • Press the button one last time when you’ve finished the designated distance.
  • The speedometer now is recalibrated itself.

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