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Old 18th March 2014, 12:42   #1
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Question About Non-Linear Speedometers

Today morning I noticed an unusual thing about in my Polo HL1.2 (P). The speedo seems to be calibrated in a different manner! Let the pictures do the talking. The speedo reads 10, 20, 30, 40, 60, 80, 100.....

Agreed that there is nothing wrong in that, but how can the calibrated distance between 30 & 40 be the same as between 40 and 60? Am I missing something or what? I m attaching the pics of speedometers from Amaze, BMW, Swift etc. None of them is like that. Can a speedometer needle have different calibration after 40 kmph or is it a misprint in my polo???


If at all, why is this like that in the first place. If for some reason, then why only Volkswagen?
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Last edited by akash_m : 18th March 2014 at 12:54.
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Old 18th March 2014, 12:56   #2
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Default re: About Non-Linear Speedometers

This is non-linear speedo. VW likes it.
1. You get better accuracy in lower range by increasing resolution
2. You "feel" good acceleration from stand-still since the needle moves faster
3. By reducing resolution in higher range, you can have accomodate the speed indication till 260 kmph, although it is not required !
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Old 18th March 2014, 14:00   #3
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Default re: About Non-Linear Speedometers

Either that, or VW speedo designer ran out of room after he got to 40. And decided to use only even numbers beyond 40.
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Old 18th March 2014, 15:59   #4
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Angry Re: About Non-Linear Speedometers

If the speedo needle is between 40 & 60, should I read the speed as 45 kmph or 50 kmph?
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Old 18th March 2014, 17:02   #5
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Default Re: About Non-Linear Speedometers

Quote:
Originally Posted by akash_m View Post
If the speedo needle is between 40 & 60, should I read the speed as 45 kmph or 50 kmph?

You should pay attention to the road..

but a valid question, each manufacturer have their own versions; w124 dont have anything below 20; i dont think there is any other reason but design and the need to be different
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Old 18th March 2014, 17:10   #6
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Default Re: About Non-Linear Speedometers

So how does the non-linear speedo work? Does the rate at which the needle moves after 40 kmph slow down?
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Old 18th March 2014, 17:22   #7
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Default Re: About Non-Linear Speedometers

Did you check the speedometer value with a GPS?
Its a strange design in my opinion ... and quite unnecessary since it would cause a person to pay extra attention to the speedo (to gauge the value) while it is doing that speed.
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Old 18th March 2014, 17:25   #8
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Default Re: About Non-Linear Speedometers

Quote:
Originally Posted by akash_m View Post
If the speedo needle is between 40 & 60, should I read the speed as 45 kmph or 50 kmph?
It will be 50, which is also our natural instinct.
You divide the increments between the end point 60 and start point 40 for that range and not beyond that.

Although it gives you sporty feel till 40 kmph since the needle moves faster, you will see suddenly the needle moves at half speed after 40 even you accelerate steadily !

I have seen a speedo with gradually reducing the resolutions on one of the concept cars but it must be quite a task to calculate the actual speed.
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Old 19th March 2014, 00:11   #9
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Default Re: About Non-Linear Speedometers

Quote:
Originally Posted by alpha1 View Post
Did you check the speedometer value with a GPS?
Its a strange design in my opinion ... and quite unnecessary since it would cause a person to pay extra attention to the speedo (to gauge the value) while it is doing that speed.
I recall my delhi-agra trip in Oct'13, the meter is fairly accurate upto 80-90km/h with a difference in speedo and gps around 1-2 km/h; speedo showing higher.

Thereafter the speedo becomes a bit optimistic
Speedo - 115km/h Gps 109 km/h
Speedo - 130km/h Gps 123km/h
speed0 - 160km/h Gps 147km/h

The observations are based upon speed displayed by MapmyIndia AVNR installed in my Polo 1.2P-CL.

I have driven i10, optra, polo back to back and the speedo design really gives a feeling of higher acceleration.

Last edited by mishraak : 19th March 2014 at 00:14. Reason: Spelling & Grammar
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Old 19th March 2014, 11:23   #10
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Default Re: About Non-Linear Speedometers

Fundamentals in how an electronic speedo works:

Speed sending
- A speed sensor in mounted near the output shaft of the transmission
- The sensor is close to teeth of gear on the output shaft
- When the vehicle moves, the teeth move very near to the sensing part of the sensor
- Each time a tooth passes from area near to the sensor, an electrical pulse in generated (changing magnetic field inducing elec voltage)
- The frequency of pulses is in direct co-relation with the speed of the vehicle (speed of output shaft)
- Thus sensor output is a chain of pulses with frequency directly proportional to the vehicle speed

Speedo
- The speed signal is communicated to microcontroller inside the speedometer, either directly or relayed through some other on-board controller
- The microcontroller drives a stepper motor controlling the needle of the speedo
- The microcontroller can be programmed to control the needle as per linear as well as non-linear scales

The basic advantage of having no linear scale as already highlighted by fellow bhpians is that we get more accurate reading at lower speeds which perfectly makes sense.

Personal opinion – Even I am not a big fan of non linear scales. :P
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Old 19th March 2014, 14:39   #11
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Default Re: About Non-Linear Speedometers

Quote:
Originally Posted by fazonater1911 View Post
So how does the non-linear speedo work? Does the rate at which the needle moves after 40 kmph slow down?
Yes.

The needle will begin to sweep half as fast (per unit speed increase) once it crosses 40 km/h (in this case).

Given that its an electric motor controlling the needle in all modern cars (unlike a mechanical linkage in older cars) this is easily possible.

cya
R

Last edited by Rehaan : 19th March 2014 at 14:40.
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Old 19th March 2014, 16:22   #12
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Default Re: About Non-Linear Speedometers

Quote:
Originally Posted by Rehaan View Post
Yes.

The needle will begin to sweep half as fast (per unit speed increase) once it crosses 40 km/h (in this case).

Given that its an electric motor controlling the needle in all modern cars (unlike a mechanical linkage in older cars) this is easily possible.

cya
R
Exactly
There is no cable
There is a speed sensor, which gives exact speed to the display.
The display can then display the speed digitally, via a dial or even speak it out if the designer so desires.
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Old 20th March 2014, 21:12   #13
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Default Re: About Non-Linear Speedometers

Quote:
Originally Posted by mishraak View Post
I recall my delhi-agra trip in Oct'13, the meter is fairly accurate upto 80-90km/h with a difference in speedo and gps around 1-2 km/h; speedo showing higher.
Difference of 1-2km/h on a GPS based speedo has multiple reasons:
  • GPS always computes speed based on time and straight line distance travelled between two consecutive points that it measures. If the roads are slightly curvy, you actually travelled a higher distance in that time than your displacement indicates and hence the car speedo indicates slightly higher speed.
  • In most GPS devices, readings would have a slight time lag and hence what you see now would be your speed 1-2 seconds ago. You can check the speeds on the GPS speedo when you slow down - it'll take a second or so before starting to decrease.
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Old 20th March 2014, 22:04   #14
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Default Re: About Non-Linear Speedometers

Quote:
Originally Posted by zenren View Post
Difference of 1-2km/h on a GPS based speedo has multiple reasons:
  • GPS always computes speed based on time and straight line distance travelled between two consecutive points that it measures. If the roads are slightly curvy, you actually travelled a higher distance in that time than your displacement indicates and hence the car speedo indicates slightly higher speed.
  • In most GPS devices, readings would have a slight time lag and hence what you see now would be your speed 1-2 seconds ago. You can check the speeds on the GPS speedo when you slow down - it'll take a second or so before starting to decrease.
Actually, most manufacturers show speeds about 5% higher than actual in most cars, because they always want to err on side of caution(higher speed shown due to lower tire pressure etc.,)
Even if you are continuously travelling in a straight line at constant speed, speedo's of most cars will show higher speed than GPS
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Old 20th March 2014, 22:52   #15
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Default Re: About Non-Linear Speedometers

Quote:
Originally Posted by anoop_lamba View Post
Fundamentals in how an electronic speedo works:

Speed sending
- A speed sensor in mounted near the output shaft of the transmission
- The sensor is close to teeth of gear on the output shaft
- When the vehicle moves, the teeth move very near to the sensing part of the sensor
- Each time a tooth passes from area near to the sensor, an electrical pulse in generated (changing magnetic field inducing elec voltage)
- The frequency of pulses is in direct co-relation with the speed of the vehicle (speed of output shaft)
- Thus sensor output is a chain of pulses with frequency directly proportional to the vehicle speed
...
Does this mean that there is no relation between calculating speed and the wheels of the vehicle? Reason for asking is, on most cases related to changing the profiles or sizes of tires are discussed, one of the most common things listed as affected are, the speed indicated by the speedometer alters if oem tire specs are altered.

Quote:
Originally Posted by tsk1979 View Post
Actually, most manufacturers show speeds about 5% higher than actual in most cars, because they always want to err on side of caution(higher speed shown due to lower tire pressure etc.,)
Even if you are continuously travelling in a straight line at constant speed, speedo's of most cars will show higher speed than GPS
If this is true, isn't this an incorrect practice, even if meant for a good cause? Instrument cluster information is supposed to be accurate to the dot and not compromised, abeit slightly, knowingly?
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