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Old 24th August 2019, 16:35   #46
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Re: Speedometer calibration: Government says that vehicle should overstate speed

This is ridiculous. Next should they even inflate mileage figures and even the amount of fuel dispensed by fuel retailers? They may even change 1kg stone weight to 800 grams to make us believe and satisfy us that we purchased more vegetables for the same money.

Whats the Bureau of Weights and Measures doing?
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Old 27th August 2019, 13:40   #47
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Re: Speedometer calibration: Government says that vehicle should overstate speed

Quote:
Originally Posted by d3mon View Post
In my Aspire, when I read the speed using OBD, it was the "accurate" speed that always matched GPS speed, while the speedo indicated version was the "doctored" version. I'm sure most car manufacturers have an algorithm to increase the speed displayed by some amount to conform to these regulations, since measuring speed accurately isn't really rocket science and cars do need to have a very accurate measure of speed for various reasons (ABS, TC, ESP etc.)
Not necessarily. Vehicle electronics use the count of wheel rotations per sample period to measure the speed, calibrated with a fixed diameter of tyre under standard recommended pressure. A worn out tyre with little tread or under inflated tyre will need more rotations to cover the same physical actual distance and will show higher speed than actual, while a oversized tyre or over-inflated tyre will show lower speed than actual. OBD measurements are based on wheel rotations, which can be further manipulated to be shown on speedometer. (actual circumference of wheel = 2 * pi * R where R is actual radius of tyre, not the rim)


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Originally Posted by pganapathy View Post
This is just so stupid. Why can't speedometers measure the speed accurately. There is no real benefit to thinking you are doing 120 when you are really doing 100. So like clothes manufacturers putting smaller labels on their clothes to make people think they are slimmer than they really are
Vehicle's speedometers typically rely on counting wheel rotations, they do not use GPS triangulation, Radar or Lidar techniques against any fixed reference point etc. to measure vehicle speed. A combination of all these could have given speedometer measurement close to the accurate numbers, however there is always some error in measurements due to technological limitations and other factors.

The police radars / lidars do have measurement errors. They are less accurate when working with higher speeds in denser traffic and other conditions than standalone measurement for a single slower moving vehicle in a straight line towards or away from radar/lidar.

For members interested in Radar/Lidar technology basics and what introduces measurement errors in external radars used by police, I would suggest the following read:

General principles: https://copradar.com/

Doppler Radar:https://copradar.com/chapts/chapt3/ch3d1.html

Operational challenges:https://copradar.com/chapts/chapt3/ch3d5.html
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Old 28th August 2019, 19:22   #48
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Re: Speedometer calibration: Government says that vehicle should overstate speed

Quote:
Originally Posted by bharatbits View Post
This is ridiculous. Next should they even inflate mileage figures and even the amount of fuel dispensed by fuel retailers? They may even change 1kg stone weight to 800 grams to make us believe and satisfy us that we purchased more vegetables for the same money.

Whats the Bureau of Weights and Measures doing?
This is not as you think Sir..Every measurement has an error which is accepted by the industry.

Depends as to upto which decimal point we are referring to!!
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Old 16th September 2019, 19:34   #49
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Re: Speedometer calibration: Government says that vehicle should overstate speed

Won't it affect the odometer reading as well?
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Old 31st October 2020, 12:07   #50
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Re: Speedometer calibration: Government says that vehicle should overstate speed

Hi.

I have some doubts regarding the tyre size and the speedometer readings.

Consider Tata Tiago. XE, XT, XZ and XZA variants are equipped with 175/65 R14 tyres while XZ+ and XZA+ variants are equipped with 175/60 R15 tyres. If we upsize the tyres of the lower variants to that of the higher variants, we get a final deviation of about 1.39% (checked using mycarhelpline.com), which is somewhat higher.

Speedometer calibration: Government says that vehicle should overstate speed-img20201031wa0002.jpg

Speedometer calibration: Government says that vehicle should overstate speed-img20201031wa0001.jpg

Some of the questions which are in my mind:

1) Do the car manufacturers tune the speedometer in accordance with the tyre size even if the car model is the same?

2) What about upsizing the stock tyres with those provided with the top model?

3) Could this be a reason for the manufacturers to provide one kind of instrument cluster in the lower variants and another one in the higher variants? (e.g) Wagonr.

4) How does the tyre size affect ground clearance?

Thanks.

(Mods, please move this post if it is irrelevant).

Last edited by Tom Sooraj : 31st October 2020 at 12:09. Reason: Spacing
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Old 31st October 2020, 14:38   #51
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Re: Speedometer calibration: Government says that vehicle should overstate speed

So the government wants us drivers to have the mental satisfaction for the perception of higher speeds on our pothole filled and congested roads.

Jokes apart, what does this ensure? Will it keep people driving under the speed limit if they think they are doing 100 kmph but actually they are at 90 kmph. Next time there is a high speed collision will they actually tell that the car was travelling at 120kmph on the speedometer and 100kmph in actual. What will this do to the testing of vehicles and the research /feedback?
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Old 31st October 2020, 20:09   #52
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Re: Speedometer calibration: Government says that vehicle should overstate speed

On this topic I got to know of a very interesting case in Indian market where in vehicles in the line up of a certain maker in India (No it isn't VW!!) seem to be showing higher FE value in ODO (by close to 4-5 km/l) compared to what users are reporting as per their real world usage experience. The case is under investigation by appropriate authorities.
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Old 31st October 2020, 20:25   #53
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Re: Speedometer calibration: Government says that vehicle should overstate speed

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Originally Posted by Carma2017 View Post
The case is under investigation by appropriate authorities.
Hope it's not Tata, my Tigor had consistent difference between real world and MID displayed mileage. But can't think of any other car maker.

P.S I have the same doubt about speedo difference between variants having different size tyres!

Last edited by giri1.8 : 31st October 2020 at 20:27.
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Old 31st October 2020, 22:48   #54
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This is not a unique Indian specific phenomena. Just about all countries in the world apply the same principle. Irrespective, car manufacturers are likely to take the same approach, just to err in the safe side of cautious. It makes a lot of sense, and normally the errors are not that big either.

So if a road speed sign says 80 km/h and everybody sticks to that speed, most cars will be traveling just a few kilometers/h less. Nobody will be braking the speed limit as long as stick to 80 on your speedo. So you wont get a speed fine, the car manufacturer doesn't get sue by you for getting a speeding ticket etc.

Your GPS will,give you a very accurate speed, as long as you are driving a constant speed. When accelerating/braking GPS speed can be wildly off.

There is no reason your speedo can not display speeds as accurately as GPS, but it will need to be calibrated as such.

Jeroen



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Old 31st October 2020, 23:32   #55
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Re: Speedometer calibration: Government says that vehicle should overstate speed

Quote:
Originally Posted by Jeroen View Post

So if a road speed sign says 80 km/h and everybody sticks to that speed, most cars will be traveling just a few kilometers/h less. Nobody will be braking the speed limit as long as stick to 80 on your speedo. So you wont get a speed fine, the car manufacturer doesn't get sue by you for getting a speeding ticket etc.


Jeroen
While I agree with the general logic here, I have seen many youtube videos where the cars are driven 5kmph( or mph) above the speed limit to compensate the speedo variance.

It's similar to my watch which is always 5 mins faster than usual time and I always try to adjust these 5 mins.
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Old 1st November 2020, 12:15   #56
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Re: Speedometer calibration: Government says that vehicle should overstate speed

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Originally Posted by SKC-auto View Post
While I agree with the general logic here, I have seen many Youtube videos where the cars are driven 5kmph( or mph) above the speed limit to compensate the speedo variance.
.
Last count there are currently some 1.5 billion cars in the world. The vast majority of its owners and drivers would not have heard about this. Even on a dedicated car forum such as this it appears new to some of our car enthusiast.

A couple of hundred, even couple of thousand YouTube videos is not the norm.

Most car owners are completely ignorant of what they would consider anorak facts at best. Out there with the need to idle your engine for a minute before shutting down. 99% of the 1.5 billion car owners out there has no clue and could not care less. And there is no evidence to suggest they are better or worse drivers, have more or less accidents, or their cars require more or less maintenance than car enthousiast.

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Old 1st November 2020, 14:01   #57
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Re: Speedometer calibration: Government says that vehicle should overstate speed

Quote:
Originally Posted by Tom Sooraj View Post
Hi.

I have some doubts regarding the tyre size and the speedometer readings.

Consider Tata Tiago. XE, XT, XZ and XZA variants are equipped with 175/65 R14 tyres while XZ+ and XZA+ variants are equipped with 175/60 R15 tyres. If we upsize the tyres of the lower variants to that of the higher, we get a final deviation of about 1.39% (checked using mycarhelpline.com), which is somewhat higher.

Attachment 2074203

Attachment 2074202

Some of the questions which are in my mind:

1) Do the car manufacturers tune the speedometer in accordance with the tyre size even if the car model is the same?

2) What about upsizing the stock tyres with those provided with the top model?

3) Could this be a reason for the manufacturers to provide one kind of instrument cluster in the lower variants and another one in the higher variants? (e.g) Wagonr.

4) How does the tyre size affect ground clearance?

Thanks.

(Mods, please move this post if it is irrelevant).
My answer is based on my professional experience in speed calculation algorithm and SW for the automobiles:

1. Yes the speed calculation is dependent on the tyre size irrespective of same car model. It is based on the no. of rotations of each wheel per second and the circumference of the wheel. Simple physics.

2. As far as speedo is concerned, for upsizing you have to keep the overall wheel diameter (alloy diameter + tyre sidewall) same.

3. That could not be the reason in most of the cases because in modern cars, the tyre diameter is a SW parameter and mostly the speed calculation is done in either engine ECU SW or braking/ESP ECU SW. So the reason for different speedometer console would be a difference in HW and it’s cost.

4. I am not sure whether suspension springs would keep the GC same even if the new tyre diameter is different.

My recommendation for upsizing would be to keep the wheel diameter same because change in diameter would affect speed calculation which in turn has an impact in many engine functions, features such as cruise control and more importantly the safety functions such as ABS, ESP, Airbag, etc. When the wheel diameter is not same as the one provided by OEM, then it is reasonable for an OEM to reject any warranty claim or insurance companies to reject insurance payouts in case of accidents.
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