[simeonovitch]

So I came across this BIS document and thought this might interest you.

The biggest takeaway is that government regulations state that a vehicle should overstate the speed by 4, 6 or 8 km/hr depending on the class of vehicle. (para 5.5)

Also has other testing requirements, accuracy requirements, criteria for approval etc.

[Comrade]

If I am reading this correctly, the document is saying that indicated speed can't be understated. Also, it can be overstated only to some extent.

Like if true speed is 100 kmph, permissable speedometer reading can be 100-114 kmph. it can't be less or can't be more.

[condor]

Speedometers already overstate speed, and I think this is the Industry norm for quite some time. One can check this with a GPS. Even the test reports published in auto mags state "indicated speed" and a measured speed.

[Dr.Naren]

Thanks for the PDF.

I was wondering why speedo error is lesser in Baleno, Grand I10 etc compared to S-Cross. S-Cross has the highest speedo error in my garage. I have verified it using GPS device named Dragy.

So the speedo error is not just in terms of percentage. It is based on different formulas for different classes of vehicles.

GPS vs Speedo (101 vs 110)

[Traveler]

In the Pajero SFX the speed was always 8 kmph more than the actual. I tested this many times.

As a corollary the kmpl figures stated by all companies should also be directly related to this. If a car is showing 8 kmph more than the actual speed then it will likely cover a kilometer earlier than actual and the odometer will show the same error.

[McLaren Rulez]

Speedometers are already calibrated to overestimate the speed. The reason is simple - if it ever underestimated the speed, you could blame the car company when you get a speeding fine.

[Senna4Ever]

Originally Posted by simeonovitch

The biggest takeaway is that government regulations state that a vehicle should overstate the speed by 4, 6 or 8 km/hr depending on the class of vehicle. (para 5.5).

That's a wrong interpretation. Same wrong interpretation has been made in the Team-BHP news post also.

Requirement is basically for two things:

• First, indicated (speedometer) speed should always be more than or equal to the true speed. (and not necessarily by 4kph, 6kph etc..)
• Second, indicated speed should not exceed the true speed by more than (for a car): 10% of true speed + 6kph.

For e.g. A true speed of 100 kph is allowed to be indicated anywhere between 100kph and 116 kph in a M category vehicle.

Therefore, to avoid breaking the rule (showing 99kph on speedo for 100kph true speed), it is already a standard practice to calibrate the speedometer for atleast 4-5% more than true speed.

[condor]

Originally Posted by Senna4Ever

, it is already a standard practice to calibrate the speedometer for atleast 4-5% more than true speed.

It is not (always) a percentage. Like @Traveller mentioned, the reading can be a fixed number over actual speed. The Safari Storme shows a 4 kmph higher speed through the range, right from around 12-15 kmph all the way to the the 100 odd kmph I have tested it, against a GPS.

[ACM]

So when we upsize the tyres we inadvertently break this rule as the speedo error in many occasions is significantly reduced and in fact the vehicle starts showing close to correct speed.

The rule though is fair in my view.

[Thermodynamics]

I always had doubt, now realized it could be true. This could also be true in other countries. One of my US friend says, different states have different speed limits and overspeeding tickets hurt. Given the size of US they often need to munch miles. He runs 9 mph above limit on interstates and he never got a ticket in 35 years of driving. I asked him why 9, he said, 10 would be a double digit overspeeding and more likely to attract ticket

[d3mon]

This is not something that just the Indian Govt. says. Request the mods to title the thread appropriately. Overstating the speed on the speedo is an internationally accepted norm, with slight variations on the actual requirements.

"The European Union requires adherence to UN ECE Regulation 39. It’s a lot of math, but the simple version is that no speedometer can read slower than the actual speed. Ever. On the high side, it’s allowed to read up to 10% above the actual speed plus four or six kilometers per hour, depending on the type of vehicle."

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.)

[pganapathy]

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

[i74js]

Dubai also permit 10% over speed for all class of vehicles beyond the stipulated limits for speed tickets. The reason though has no linkage with the error in speedometer but it's the tolerance embedded in system for breach upto 10%.

Even in Delhi where I was challenged for speed of 64 kmph few years back (as per the speed camera value), the cop said had you been under 60 we would have just stopped and warned you without issuing a ticket for over-speeding (speed limit was 50 kmph on that stretch).

But I fail to understand the reason for embedding the error. It's like keeping the watch / clock fast by few minutes which serves no purpose as out mind gets trained within few days and the impact gets adjusted / appended in our routine; more or less the same will happen with automobile owners.

[hemanth.anand]

Originally Posted by pganapathy

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

This regulation has been there since many years and it is fair regulation IMO.

Natural variations doesn't allow accurate and consistent measurement of speed. The tyre diameter is one such major parameter. Tyre diameter varies a lot due to inflation pressure, tyre size, wear. Hence, speed calculation can’t be accurate and consistent over time.

So the regulations have set the following 2 important rules
1. The indicated speed must never be less than the true speed.
This helps people to be within speed limits on the road. If because of the tyre diameter, the indicated speed was less than actual speed, there would be lot of speeding violation cases.
Another good thing will be human tendency is to be happy with a higher “perceived” speed (or displayed speed).
However Manufacturers can’t display a very high speed to keep their customers happy. That’s where the second rule comes in.

2. The indicated speed must not be more than 110% of the true speed plus 4 km/h.
For example, at a true speed of 80 km/h, the indicated speed must not be more than 92 km/h (88+4).

[KL01toKA03]

Are speed radars also calibrated, by law, using a similar logic? That is, is the displayed speed always less than or equal to the actual speed?

That is also required to make it really fair.