[haisaikat]

Last week while driving the Innova from Kolkata to Puri and back I noticed this things specifically, while Speedo shows 85 km/hr the Google map on mobile shows 7-8 km/hr lesser reading and this difference is consistent.

I think my older car 2007 Hyundai Getz showed much lesser difference of 1-2 km/hr max.

On the contrary a 2009 Hyundai i20 I drove of a friend seemed to have higher difference as I always felt how come it attains 60km/hr so quick when looking outside the window I felt always it's lesser.

I believe speed should be accurate, or else I feel misguided especially for responsible drivers.

[headers]

Originally Posted by 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.

Yes no two instruments will measure the exact same speed. There has to be an acceptable error for them to qualify the process. And GPS BTW need not be accurate too!

[condor]

Has anybody checked the indicated speed vs vLog speed published in the reviews by auto-mags ? Dont remember which one(s) - could be OD.

Originally Posted by headers

Yes no two instruments will measure the exact same speed.

Good, good you mentioned this. I was thinking all vehicles must show same over-clocked speed.

[Traveler]

So does this also mean that apart from the per kilometer figures that will actually show more fuel mileage per kilometer even the 0 to 100 speeds are wrong.

[Balboa]

The worst(?)offenders are VW group where the speedometer shows much more than the actual speed. Noticed in my Polo speedometer always showed 10% more than the actual speed. My wife's Santro automatic too I believe is very optimistic. It shows that I am doing 80 kmh when I am sure there is no way I am doing that speed. My new Honda City on the other hand seems to be very accurate. In fact I don't think there is any variation at all with the actual speed or at least very very minimal

[Vikki@Hyderabad]

So what this means is economy figures, service intervals and warranty coverage are all less than actual.

As a consequence of overstating speed, distance traveled also gets overstated. For example, in my Figo (not anymore for 2 years), an indicated 100 kmph was ~92 kmph on GPS, and every 100 kms traveled (measured by milestones) would show ~104 kms on the Odo. And this, in turn, overstated mileage figures by at least ~1 kmpl. However, after installing 185/70 tires, indicated 100 kmph was about ~98 kmph on GPS and every km traveled physically was about ~0.97 kms on the odo. And also, as a result, my economy figures dropped by about ~1.5 kmpl after changing the tires at 45K kms. Still happy though, as I had an overall mileage of 16.7 kmpl throughout its 70K kms.

When I was a kid, my dad used to tell me that the milestones (misnomer actually) on a highway are 1.1 kilometers apart as measured by his scooter's odometer. Found out that almost the inverse is true after all these years.

Disclaimer: All evidences presented are anecdotal in nature.

[Motard_Blr]

The speed displayed by a speedometer depends on the tyre diameter and the error that is inherent in the measurement.

For example, a 195/55R15 tyre the overall diameter is 595 mm. By the time it is replaced die to wear, let's say that of the total tread depth of 6 mm, only 5 mm is used up. This means that by the time this tyre is replaced the diameter would be (595 - 5 - 5) = 585 mm. This is about 1.7% smaller than a new tyre. Because this is quite small and also because not all tyres of the same indicated size actually have the same diameter, it can be ignored.

What cannot be ignored is tyre flattening due to the vehicle weight. This could be about 10 mm. So for the tyre I have considered, 10 mm reduction in radius represents 20 mm in diameter and a reduction of 3.3%. This error will always be positive because a reduction in radius will cause the indicated speed to be higher. I think that manufacturers do correct for this error to some extent.

The speedometer like any other measurement device is not 100% accurate. The typical allowable error might be about ±3% (my estimate). This means that what the speedometer is displaying might actually be:

(+3.3% tyre flattening + 3% speedometer error) = +6.6% on the plus side.
And (+3.3% - 3%) = 0.3% on the negative side.

The total error is 6.9% rounded up to 7%.

Additionally, the rules demand that speedometers error should always be on the plus side. In order to conform to this rule, the error is shifted to the plus side i.e., the entire error of 7% is positive.

So if the true speed is 100 km/h the indicated speed might be as high as 107 km/h. Remember that this is just what is allowed by the rules and a real speedometer may not be this inaccurate.

[anoop_lamba]

For all the friends wishing for exact speed to be displayed, no measurement system can be precise or 100% accurate. There is always a tolerance in accuracy of measurement, especially when multiple components are involved in a measurement system like in the case of speed sensing.
The law simply states that tolerance need to be on the positive side such that indicated speed is more than actual speed with max difference defined.
It is upto manufacturers how close they want the two values to be but again given the possible variations in the sensing system, the difference kept is significant to ensure the law is met.
Please note that the same standard in applicable to vehicles with old rotating speedo cable type speedometers too.

[superbad]

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

Say you are driving at 80 kmph (indicated, with no speedo error) on a road with similar speed limit. Even a momentary increase in speed is enough for the cops to catch you & fine you. However if the indicated speed (80) is higher than actual speed, there's some leeway for you if you temporarily increase speed & stay on the safe side of the law.
One more reason for overstating speed is to make people think they're traveling faster, so that they back off a bit.

[pganapathy]

Originally Posted by superbad

Say you are driving at 80 kmph (indicated, with no speedo error) on a road with similar speed limit. Even a momentary increase in speed is enough for the cops to catch you & fine you. However if the indicated speed (80) is higher than actual speed, there's some leeway for you if you temporarily increase speed & stay on the safe side of the law.
One more reason for overstating speed is to make people think they're traveling faster, so that they back off a bit.

Sorry, I don't think this makes sense. If your indicated speed is 80, you are unsure what your actual speed is. It might be 70 or 79, depending on how accurate the speedo is. Therefore, the margin for error you have is an unknown. On the other hand, if the speedo is accurate, you know exactly what speed you are at and your margin for error is

[VijayAnand1]

Interesting topic.

The key point behind an optimistic speedo than a pessimistic one is simply, drive/ride safely. Irrespective of opti/pessmistic speedo, we have dimwits lathered in plenty on the roads. When some can't see the road properly and drive, how would they see the speedo reading and then take corrective actions -- on a lighter note.

95% of the time, drivers don't bother to look at the speedo unless it's a freeway express or highway or the sorts. Rest of the time, their speed, their driving conditions depends on their mood, the ambiance and what not.

When people can't read the road signs where it says slow down, where it says 40 kmph, 30 kmph so and so forth, yet still speed up like there's no tomorrow, the speedometer is just another meter showing a parameter they least care about.

But, manufacturers have their own protocol to follow, their own safety mandate which in turn helps us be a safer motorist out there.

Cheers!
VJ

[superbad]

Originally Posted by pganapathy

Sorry, I don't think this makes sense. If your indicated speed is 80, you are unsure what your actual speed is. It might be 70 or 79, depending on how accurate the speedo is. Therefore, the margin for error you have is an unknown. On the other hand, if the speedo is accurate, you know exactly what speed you are at and your margin for error is

Across the board, for all vehicles, on an average speedo always shows 5-7% excess by default, from factory. More importantly, how many people really know this fact? Aside from auto enthusiasts, not many. Thus, for them they might be cruising at 80 kmph indicated, however actual speed would be 72-75 kmph, thus with adequate margin of error.

[wrongturn]

Government: Calibrates the speedo to overstate the speed by 10km/h, hoping everyone one will drive slow.

My Mother: Forwards the clock 5 minutes to overstate the time, hoping everything will be done on time.

Me:

[Yieldway17]

Overenthusiastic speedometers and no mention of Aprilia scooters? It is widely known that Aprilia scooters in India overstate speeds by even up to 20kmph.

[K_Drive]

Today I was comparing my speedometer reading with Google Maps. I was driving Wagon R 2008, Tyre size changed from 145/70/13 to 155/70/13. Speedometer reading was 70KM/H. Google maps was showing 65. Since there is change in tyre size than the factory one, the error in reading on speedo should be 2km/h. So the actual speedo reading should be ~72km/h. That is 10% higher than the actual speed. What my point is, we have some calibration in place even before 11yrs.