What does the odometer and speedometer of an automobile measure?

What does the odometer and speedometer of an automobile measure

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The instrument cluster is an important part of the car because it contains the instruments that help you drive.  

These include the speedometer and odometer. The speedometer tells us the speed of the car and the odometer indicates the number of kilometers traveled. 

This may be confusing to some, so let’s have a closer look at these two important instruments.

A speedometer tells us the speed of the vehicle at any given time. The odometer’s main function is to keep track of the total number of kilometers the vehicle has travelled and is non resettable.

Odometer is a meter which shows how much distance the automobile has run. It is in kilometers (km) in India and many other countries or in miles as in the USA. Odometer can be of two types- analog and digital.

Speedometer on the other hand, measures instantaneous speed of the vehicle. Instantaneous speed means the speed at that given moment. The reading you see is the speed of the vehicle at that very moment. Not average, nothing else. It can be read as kilometers per hour (km/h) or miles per hour (mph) as per the country. Speedometer can also be digital or analog like the odometer.

Now let us study odometer and speedometer in detail.

 

 

What does the odometer of a vehicle measure?

In the dashboard, which is beneath the steering wheel, comes an odometer which shows the distance the car or bike has run for.

Odometer comes in handy especially if you are about to buy a used car or sell your old one. From the odometer we can calculate the mileage of the car which can be useful while buying a used car. Also, we will be aware about the distance the car has run for and can predict the engine condition.

In this blog, we will also learn to predict the mileage of a vehicle using the odometer reading.

 

Different purposes that the odometer serves:


The resale value of a vehicle

The resale value of a particular vehicle is determined by two main factors- the condition of the vehicle and the odometer reading. For example, if a car has a new looking appearance but the odometer reading is high, it will have less value than a similar condition car with less odometer reading.


Mileage calculation

We will talk about this in detail in the next section. Using the odometer we can also calculate fuel economy in km/liter, which can give us a rough idea of how much fuel should be filled before going on a particular journey.


Prediction of life of an existing tyre

The tire is a component which undergoes wear and tear during everyday running. A tire from a renowned manufacturer can typically last for about 50k kms. So, you can check the odometer after every replacement and after 50k kms is completed in the odometer, then change the tire. This way, you won’t have to check the tire condition time and again. 


Servicing and Maintenance

This serves an important aspect. Your bike or car needs to be serviced after a specific running distance. You can note down your odometer reading after each service so that you can go for servicing after every recommended distance completion.

Many modern day bikes have this feature inbuilt. Bikes like KTM and many scooters from Hero show the distance left before next service. They also alert you when you’ve covered the distance to go for a servicing.

Now let us learn to calculate the mileage from the odometer and then at the end of this section we will inform you about various kinds of odometer tampering which you should be aware of while purchasing an old car or bike

One of the easiest ways to calculate mileage from the odometer is to first empty the fuel tank completely. The last bit of fuel can be emptied by keeping the ignition on for some time.

Then fill one liter of fuel into the fuel tank of your car or bike and check the odometer reading. Now, drive the vehicle for as long as it can run on the fuel. Note this reading again.

Subtract the initial reading from the final reading and this is the mileage of your vehicle. The unit used is kmpl i.e., kilometers per liter. 

This is a bit of a hassle to do, so nowadays many cars and many bikes too come with inbuilt mileage calculators and distance to empty. You can find out how much more distance your vehicle will run on the fuel beforehand. This is very convenient

Another important aspect to talk about odometers is the fraud done with it. Odometer serves a crucial role in the value of a second hand car. 

There is a fraud which many used car dealers do called odometer rollback. Suppose the car has originally run 60k kms but the dealer may commit this fraud and make the reading as 40k kms and quote for a higher price. As this is a fraud and is used to con people, it is punishable in many countries.

In the past, rolling back simply meant, car dealers manually rolled the numbers back in a manual meter.

Sometimes the odometer cluster is simply swapped with a newer one which shows low odometer reading.

Sometimes it is more hitech and in digital meters, a device is used to reset the odometer count.

 

Here’s a way to check for odometer rollbacks:

A very easy and convenient way is to ask the dealer for the complete service history of the car or bike. In service history, date and odometer is mentioned and stamped. You will get to know about the readings on the serviced date and so can easily predict if the meter being shown to you is fake or real. For many cars, proper service history is not available. In those cases, read on to know what to do.

Make a close inspection of the brake, accelerator and clutch pedal. Also the steering wheel. The pedal markings should not be faded if the odometer shows less than 40k kms reading. The steering wheel leather will also be fresh. But if these show signs of wear then the car has covered more distance.

Second thing is to check for the tires. A typical tire will last for about 50k kms. If the car you intend to purchase shows abou 40k kms and also the tire looks new then probably the tire was replaced due to wear off. Which in turn implies more running distance.

All in all, an odometer rollback cannot be reversed but you can get a hint if your odometer was rolled back.

 

What does the speedometer of an automobile measure?

Speedometer shows the speed of a vehicle at any given instant. This is called instantaneous speed. In modern times, digital speedometers are more in use which can show more accurate reading. A speedometer is generally located on the dashboard of a car or instrument cluster of a bike. 

If you are stopped by a police officer then only instantaneous speed is useful and not average speed. It is basically showing how fast the wheels are turning. 


Now how does the speedometer work?

Mechanical speedometers are linked with the gearbox output shaft. Output shaft provides the output to the wheels and lies downstream of the gearbox, so it is independent of gear changes. Whatever speed is being provided by the output shaft will be shown in the speedometer. The speedometer cable is attached to this output shaft.

 

Construction of a speedometer

With the speedometer, a magnet will be linked. Over this magnet will lie a speed cup. It will just stay above without any physical connection. With this speed cup, an indicator rod is connected and with this rod a hair spring. With this indicator rod, indicator housing is connected which shows us the speed outside in the speedometer. 


How does this construction work:

When the output shaft rotates, the speedometer cable connected to it also rotates. This cable in turn will make the magnet rotate This magnet will form a magnetic field and attract the metallic part of the speed cup. Due to this, the speed cup will be deflected and the indicator rod will rotate and deflect the hair spring. This indicator rod will move the needle in the indicator housing to show you the speed with which the vehicle is moving.

This was the physics part underlying. 


Here are some common problems with speedometers:


Speedometer inaccuracy:

Speedometers can show an inaccuracy of upto plus or minus 4%. This means when you are going faster it will depict a bit of less speed and vice versa. This can be caused by improper inflation of the tires. If you have over inflated or under inflated the tires then this problem may come into effect. So, inflate your tires to the recommended pressure.

This may also be caused by worn off tires. 


How to check for accuracy?

Take a stopwatch. As you pass a kilometer marker on the road, turn on the stopwatch and travel till the next marker. As you arrive at the next marker, stop the speedometer. The second hand reading should be equal to the speed shown in the speedometer. Also, you can visit a mechanic to check for inaccuracy.

 

Odometer and Speedometer not working?


Causes and fixes:

If the odometer or trip meter is not spinning in your vehicle at all, then here is the quick fix.

Back in the old days, cars had a spinning cable which was attached to the wheels or transmission. So, in old cars, normally non functional odometers can be due to the problem with the cable. These cables can’t be repaired and need changing.In this type of odometers, if you reverse the car then the reading will come down.

But in today’s vehicles, a completely different approach is followed. A device called a speed sensor is used for sensing the speed. In this setup a mechanical signal is transformed into an electric signal. Let me explain more clearly.

In this speed sensor, one head spins and is connected to the transmission and this is the mechanical signal. This then gets converted into an electric signal and gives the reading on the dashboard. This is used for both digital and analog odometers. In this setup, if you reverse the car, it will increase the odometer reading.

As this is mechanical, the speed sensors wear out. Generally, you will get a check engine light if it wears out. Most importantly, your speedometer and odometer won’t work. So, whenever your speedometer or odometer don’t work, replace the speed sensor. 

It is located underneath the car. So to replace it, go under the car and find the transmission. Here you will find the speed sensor. Speed sensors are easy to replace with only one bolt holding it. 

Replacing the speed sensor is a perfect fix if both the speedometer and odometer are not working.

But if only the odometer stops working then surely the gears which turn the odometer numbers have worn off. These gears are made of plastic and are brittle so can wear off or the teeth may break. Replacing the gears will fix the issue.

For this, open the dashboard compartment. This may seem intimidating but is not. Anytime you’re working in the dashboard, make sure to remove the negative terminal from the battery so that nothing sorts out by mistake.

After opening the dashboard with proper tools, you need to remove the whole gauge cluster. Now remove the top glass cover gently and pop out the odometer board. Here underneath the odometer you will have a motor and some gears beside it. Gently take out the motor and replace the motor head gear. Also, replace all the gears present in the setup. Just remove them and fit the new ones in place. That’s it.

In this blog, we wish we could provide you with all the practical  knowledge available regarding odometer and speedometer and what important aspects they serve. Keep reading our blogs to learn new things everyday about your car and bike.

Chandra Prabha Ghosh

Chandra Prabha Ghosh

An automotive enthusiast of both bikes and cars. Pop genre fan. Likes to explore new technologies like metaverse etc.

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