What is the difference between speed and speedo ??

santosh.s · 25th July 2006, 13:25

Quote:

Originally Posted by supremeBaleno

Speedo error !!! This is one term that keeps popping up a lot on tbhp. Have a couple of queries about this. Experts, your views solicited.

1. My thinking is that speedo error happens only on changing the stock wheels/tyres. Or is it that a stock car will also have speedo error ? If so, what is the reason for such error ?

Apart from various reasons mentioned by others, here are a couple of other reasons behind the error- 1. tyre circumference keeps on reducing as it wears out over time. 2. actual distance travelled for every revolution of the wheel may not be exactly equal to its circumference depending upon air pressure and load condition

Even Shan2nu's formula is prone to 2nd error, and the first one also if you calculate the circumference based on tyre size instead of measuring the actual size...assuming RPM meter is quite accurate!

Shan2nu · 25th July 2006, 13:32

Quote:

Even Shan2nu's formula is prone to 2nd error

Yes it is, and thats why......i always mention (+/- 2%) at the end.

Shan2nu

maxbhp · 25th July 2006, 13:35

Quote:

Originally Posted by tsk1979

The speed gun will tell you your current speed. Somebody has to stand outside while you approach him, or your copassenger has to aim at a stationary object right in front of you.

Yeah and it costs just 500 Rs...dont need 25K like a GPS . I was doing around 85 , I was caught by Highway Police ...they asked me what speed I was going I told them 85 ...they said "wrong !, you were doing only 74 but legal limit is 70 so be careful and for measuring your speedo correctly please pay a sum of 500rs to Indian Gov."

Shan2nu · 25th July 2006, 13:56

Quote:

Yeah and it costs just 500 Rs...dont need 25K like a GPS . I was doing around 85 , I was caught by Highway Police ...they asked me what speed I was going I told them 85 ...they said "wrong !, you were doing only 74 but legal limit is 70 so be careful and for measuring your speedo correctly please pay a sum of 500rs to Indian Gov."

Wow, 85 when you were doing 74? What car/bike was this?

Shan2nu

santosh.s · 25th July 2006, 14:41

Quote:

Originally Posted by Shan2nu

Yes it is, and thats why......i always mention (+/- 2%) at the end.

Shan2nu

Did you mean pressure/load won't cause errors in excess of 2%? consider a wheel diameter of 600mm (typical for most sedans), the standard radius becomes 300mm. even if there is extra deflection of just 1/2 inch, the error in effective rolling radius will be well above 4%. Compact cars have even smaller wheels, where the error can be more for the same amount of deflection.

Another correction, baleno vxi size is 185/65R14, which if off by just 0.32% as compared to lxi, not 185/60R14 (2.8% off!) as mentioned in this thread.

esteem_lover · 25th July 2006, 16:32

it is not as if the car manufacturers cannot build precision components and also it is not to satisfy ur fancy for fast driving. this speedo error is basically meant only to show better fuel efficiency. the good old cars those days had pretty accurate speedos and odos. the difference is more in our bikes these days

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condor · 25th July 2006, 17:27

Quote:

Originally Posted by esteem_lover

.. speedo error is basically meant only to show better fuel efficiency.

Dont think so - the odometer itself looks quite accurate, as seen when you compare the odo readings to milestone markers on the highways.

Also, test reports by mags like OD have a lot of detail, and this would have been spotted very easily. If the speedo error had affected the odo readings, then there would have been much more noise - the auto companies wouldnt have got away with something like that. It is also much easier to compare odo readings - take two vehicles of diff companies and travel the same distance, and compare the readings.

But it kind of makes sense to show a higher speedo reading than actual speeds, atleast to keep the driver happy.

Shan2nu · 25th July 2006, 17:46

Quote:

Did you mean pressure/load won't cause errors in excess of 2%? consider a wheel diameter of 600mm (typical for most sedans), the standard radius becomes 300mm. even if there is extra deflection of just 1/2 inch, the error in effective rolling radius will be well above 4%. Compact cars have even smaller wheels, where the error can be more for the same amount of deflection.

So far it hasn't. I've checked it with the GPS figures mentioned in mags (for a given rpm) and they have always been around 2%. My car shows 98 at 7100rpm in 2nd, GPS device showed 98 and my calculations show 100.

So in my case, the speedo seems to be more accurate than the calculation. LOL

Ofcourse, if someone is running on underinflated tyres, the error would increase but how do you decide by what %? It could be more than 4%.

It's impossible to get accurate figs.........and all online calculators acknowledge this fact. I'm happy to keep it to 2%.

Shan2nu

DaPilot · 25th July 2006, 23:12

Wow this is news to me... is this contained to Indian manufactured cars only?

I can tell you its definitely not a practice in the west, I have a portable gps unit and I’ve never noticed any speed discrepancies in any of my cars or my friends, in fact this almost sounds like an easy lawsuit if someone proves that the Speedo meter is intentionally tampered (for what reason it doesn’t really matter).
The whole concept of posted speed limit can be thrown out of the window... why post a higher speed limit if you don’t want drivers to drive at that speed. This just doesn’t make sense... am I missing something?

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tsk1979 · 25th July 2006, 23:20

Actually its margin of error.
There is no such thing as showing actual speed.
For example, suppose you have less air in your tires. You run at 80kmph but your speedo will show 84.
If suppose your manufacturer builds the speedo that at 28psi at 25 degree C, they speedo will show actual speed.
Its a hot day and due to high speed your tires are running 34PSI. Higher diameter. Which means you are doing 84kmph with speed showing 80. You can get a ticket for this.
Hence the margin for error. No matter where, your speedo is going to show speeds slightly higher than actual speed.
I used a portable gps in the US. The speedo was showing 85mph on the freeway while the gps was showing 82mph. Therefore this margin of error is also there in US cars. As long as the error is on side of caution, i.e. speedo shows slightly higher speed, its okay, you wont get a ticket, But suppose the speedo is showing a lower speed and you get a traffic ticket, then the manufacturer is in trouble.
The only way to make an accurate speedo is to use special timing devices used by auto mags during testing(GTEC Pro etc.,). Something connected to the wheels and depended on wheel diameter is never going to be accurate.

Shan2nu · 25th July 2006, 23:37

Quote:

Wow this is news to me... is this contained to Indian manufactured cars only?

Nope, every country has laws that allows a certain % of error. Read the para below.

IN THE REAL WORLD
People are forever claiming how fast their cars go based on speedo readings. Most speedos read way fast - the law allows a 10% error and most manufacturers set speedos somewhat fast so that you won't get get done for speeding and then sue them. I have tested a number of cars and the average error is about 5% to 7% fast - i.e. when the speedo shows 100 mph you are really doing about 93 to 95 mph. Many magazines do a speedo accuracy test as part of their report - have a look at Autocar tests or similar.

Source - http://www.pumaracing.co.uk/topspeed.htm

Shan2nu

DaPilot · 25th July 2006, 23:39

Thanks for clarifying! I guess it makes sense now... I will definately keep a closer eye on the GPS now

but it would really suck if I got a ticket for speeding when I did not (I am usually 10/15 above the speed limit - but thats besides the point lol)

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DaPilot · 25th July 2006, 23:43

Point taken... Thanks for the enlightment

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karan561 · 7th January 2007, 17:30

Espescially in TATA cars , they adjust quite a decent speedo error ....

Ram · 7th January 2007, 19:28

Well:

So it stands to reason that tires with the smallest original diameter,
like the Maruti 800's 140/70R12
and the Tata Ace's tires and Autorickshaw and Scooter tires
will show the most speedometer error due to tire wear.

Again low profile racing tires which reduce overall dia. despite large metal rims
will show more speedo error than tires with higher outer dia. with the same metal rims.

The key is:
what percent of the tire height was worn away?
This will be more for smaller tires than for example, Volvo B7R bus tires.

25th July 2006, 16:32	#21
esteem_lover Senior - BHPian Join Date: Jul 2006 Location: Madras/Py Posts: 7,556 Thanked: 502 Times View My Garage	it is not as if the car manufacturers cannot build precision components and also it is not to satisfy ur fancy for fast driving. this speedo error is basically meant only to show better fuel efficiency. the good old cars those days had pretty accurate speedos and odos. the difference is more in our bikes these days Note from support team: Do paste the entire quote. You can take lines as what you want to high-light. Please make this as a practise. Thanks. Last edited by Rudra Sen : 26th July 2006 at 10:16.
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25th July 2006, 23:12	#24
DaPilot BHPian Join Date: Mar 2006 Location: Newport (JC), NJ - USA Posts: 187 Thanked: Once View My Garage	Wow this is news to me... is this contained to Indian manufactured cars only? I can tell you its definitely not a practice in the west, I have a portable gps unit and I’ve never noticed any speed discrepancies in any of my cars or my friends, in fact this almost sounds like an easy lawsuit if someone proves that the Speedo meter is intentionally tampered (for what reason it doesn’t really matter). The whole concept of posted speed limit can be thrown out of the window... why post a higher speed limit if you don’t want drivers to drive at that speed. This just doesn’t make sense... am I missing something? Note from support team: Do paste the entire quote. You can take lines as what you want to high-light. Please make this as a practise. Thanks. Last edited by Rudra Sen : 26th July 2006 at 10:11.
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25th July 2006, 23:20	#25
tsk1979 Team-BHP Support Join Date: Feb 2005 Location: San Jose, CA Posts: 23,717 Thanked: 22,825 Times View My Garage	Actually its margin of error. There is no such thing as showing actual speed. For example, suppose you have less air in your tires. You run at 80kmph but your speedo will show 84. If suppose your manufacturer builds the speedo that at 28psi at 25 degree C, they speedo will show actual speed. Its a hot day and due to high speed your tires are running 34PSI. Higher diameter. Which means you are doing 84kmph with speed showing 80. You can get a ticket for this. Hence the margin for error. No matter where, your speedo is going to show speeds slightly higher than actual speed. I used a portable gps in the US. The speedo was showing 85mph on the freeway while the gps was showing 82mph. Therefore this margin of error is also there in US cars. As long as the error is on side of caution, i.e. speedo shows slightly higher speed, its okay, you wont get a ticket, But suppose the speedo is showing a lower speed and you get a traffic ticket, then the manufacturer is in trouble. The only way to make an accurate speedo is to use special timing devices used by auto mags during testing(GTEC Pro etc.,). Something connected to the wheels and depended on wheel diameter is never going to be accurate.
	() Thanks

25th July 2006, 23:39	#27
DaPilot BHPian Join Date: Mar 2006 Location: Newport (JC), NJ - USA Posts: 187 Thanked: Once View My Garage	Thanks for clarifying! I guess it makes sense now... I will definately keep a closer eye on the GPS now but it would really suck if I got a ticket for speeding when I did not (I am usually 10/15 above the speed limit - but thats besides the point lol) Note from support team: Do paste the entire quote. You can take lines as what you want to high-light. Please make this as a practise. Thanks. Last edited by Rudra Sen : 26th July 2006 at 10:06.
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25th July 2006, 23:43	#28
DaPilot BHPian Join Date: Mar 2006 Location: Newport (JC), NJ - USA Posts: 187 Thanked: Once View My Garage	Point taken... Thanks for the enlightment Note from support team: Do paste the entire quote. You can take lines as what you want to high-light. Please make this as a practise. Thanks. Last edited by Rudra Sen : 26th July 2006 at 10:07.
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7th January 2007, 17:30	#29
karan561 Distinguished - BHPian Join Date: Aug 2006 Location: Mumbai Posts: 5,068 Thanked: 22,247 Times View My Garage	Espescially in TATA cars , they adjust quite a decent speedo error ....
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7th January 2007, 19:28	#30
Ram Senior - BHPian Join Date: Dec 2005 Location: Singapore, Mumbai, Nagpur Posts: 2,166 Thanked: 219 Times View My Garage	Well: So it stands to reason that tires with the smallest original diameter, like the Maruti 800's 140/70R12 and the Tata Ace's tires and Autorickshaw and Scooter tires will show the most speedometer error due to tire wear. Again low profile racing tires which reduce overall dia. despite large metal rims will show more speedo error than tires with higher outer dia. with the same metal rims. The key is: what percent of the tire height was worn away? This will be more for smaller tires than for example, Volvo B7R bus tires.
	() Thanks