Odometer v Speedometer

ABEasy · 26-07-2015 10:49pm #1

Bit of a random question, but hopefully someone knows.

So, with European registered cars the speedo is set a few kmph more than the actual, eg. Speedo says 100kmph but actual speed 96kmph. Does the odometer read off the speedo? I mean after 1hr driving at 100kmph does the odometer increase by 100km or say 96km per the above example and if so are all European cars odometers overstating the actual distenance travelled?

OldMrBrennan83 · 26-07-2015 10:52pm

This post has been deleted.

CIP4 · 26-07-2015 10:53pm

I've heard before that the odometer read actual distance travel of the speed sensor in the gearbox or whatever so will be accurate and the speedometer just rounds it up a small bit then for safety reasons. So the odometer does not read the distance traveled of the speedometer.

cython · 26-07-2015 11:09pm

Given that the margin for error is built into speedometers (at least in part) to account for variations in tyre pressures, wear, sizes, etc. there is likely no more accurate measure available to the odometer (since the tyres are turning a fixed amount per distance, whereas nothing in the gearbox is), though as noted there may be an additional surplus added on to the speedo figure above and beyond the measured error to make sure. I guess cruise control on a motorway for an hour, or even 15/30 mins would be the best way to check!

visual · 26-07-2015 11:12pm

Both are off by the same %

slimjimmc · 26-07-2015 11:13pm

It's an international agreement that applies in many countries worldwide that your speedometer must not read less than the true speed and not more than 110%+6km/h at certain test speeds 40 km/h, 80 km/h and 120 km/h.
UNECE Regulation 39 (PDF)

Odometers are not mandatory in cars so is not regulated. Belguim have proposed an amendment to regulate odometers (PDF)

Eric Cartman · 26-07-2015 11:15pm

since its illegal for a speedometer to under-read , and since most of them are based on a mechanical clock spring being driven by a small motor, the margin for error is built in under the basis that the clock spring may wear a bit over time , at some point towards the clusters dying day it will probably end up bang on , but its merely a compensation for the analog nature of the speedometer and the odometer would have the accurate reading.

slimjimmc · 26-07-2015 11:16pm

It's an international agreement that applies in many countries worldwide that your speedometer must not read less than the true speed and not more than 110%+6km/h at certain test speeds 40 km/h, 80 km/h and 120 km/h.
UNECE Regulation 39 (PDF)

Odometers are not mandatory in cars so is not regulated. Belguim have proposed an amendment to regulate odometers (PDF)

CIP4 wrote: »

I've heard before that the odometer read actual distance travel of the speed sensor in the gearbox or whatever so will be accurate and the speedometer just rounds it up a small bit then for safety reasons. So the odometer does not read the distance traveled of the speedometer.

That has been my experience based on sat nav readings. The odometer and instantaneous speed from the trip computer matched the sat nav reading quite accurately but the speedometer was always showing higher.

George Dalton · 26-07-2015 11:54pm

I agree with the above. In my experience the odometer is accurate even though the speedo over-reads.

Alun · 27-07-2015 9:36am

Eric Cartman wrote: »

since its illegal for a speedometer to under-read , and since most of them are based on a mechanical clock spring being driven by a small motor, the margin for error is built in under the basis that the clock spring may wear a bit over time , at some point towards the clusters dying day it will probably end up bang on , but its merely a compensation for the analog nature of the speedometer and the odometer would have the accurate reading.

Clock spring????? Maybe if you're talking about a 1959 Morris Minor.

'Analogue' speedo dials nowadays are driven by digital stepper motors, so no 'wear' possible.

HandsomeDan · 27-07-2015 12:05pm

Ode is accurate.

Speedo is accurate but as tyres wear it over-reads.

slimjimmc · 27-07-2015 2:05pm

HandsomeDan wrote: »

Ode is accurate.

Speedo is accurate but as tyres wear it over-reads.

Tyre wear is a negligible factor, contributing about 2% of an overread.
As I linked earlier speedometers are allowed a much higher tolerance. Some are more accurate than others. Mine reads about 55 when doing 50km/h and 128 when doing 120km/h.

Cee-Jay-Cee · 27-07-2015 3:37pm

Eric Cartman wrote: »

since its illegal for a speedometer to under-read , and since most of them are based on a mechanical clock spring being driven by a small motor, the margin for error is built in under the basis that the clock spring may wear a bit over time , at some point towards the clusters dying day it will probably end up bang on , but its merely a compensation for the analog nature of the speedometer and the odometer would have the accurate reading.

Sorry but you are decades behind in your knowledge of technology. Clock springs haven't been used since possibly the 70's/80's!

CiniO · 27-07-2015 4:32pm

visual wrote: »

Both are off by the same %

In my experience they are not.
All my cars had very precise odometer, and overreading speedometer.

Eric Cartman · 27-07-2015 6:23pm

Cee-Jay-Cee wrote: »

Sorry but you are decades behind in your knowledge of technology. Clock springs haven't been used since possibly the 70's/80's!

Alun wrote: »

Clock spring????? Maybe if you're talking about a 1959 Morris Minor.

'Analogue' speedo dials nowadays are driven by digital stepper motors, so no 'wear' possible.

this is a stepper motor from an e39 / e53 gauge cluster, not exactly brand new, but modern enough tech http://www.aliexpress.com/item/1PCS-Stepper-Pointer-Motor-for-BMW-E38-E39-E53-M5-X5-Instrument-Cluster-Gauges/32283790636.html these are still used in newer gauges but these are the ones I'm most familiar with and could find a pic of easily.
$(KGrHqFHJEYE91+mLTCyBPhGMF8S)!~~60_35.JPG

$(KGrHqFHJEYE91+mLTCyBPhGMF8S)!~~60_35.JPG

if you notice the clock spring on the front of the shaft there, this is used to create the smooth motion we're used to with gauges as digital steppers can be quite jerky when setting position, now over time the resistance this spring provides against the gauge returning to 0 changes, as does the strength of the magnets inside the motor , these will be minor changes over time, but at the risk of them wearing , auto makers put readings 5km/h ish over to compensate for this, making them 100% dead on would render any speedometer illegal in the event of a fault, this is simply a margin of error method that allows for minor changes in manufacturing standards, electrical issues (varying battery voltage, speed sensor degradation, signal interference, or alternator faults could even impact)

the clock spring , while not the essential piece it once was, is very much still there and not 'from the 70s'

Alun · 27-07-2015 8:03pm

But stepper motors are capable of stepping in both directions and while I can imagine that spring might be useful as a means of damping down small oscillations as the motor steps, what would be the use of a self return spring in that case? A stepper motor will always step by the number of steps it's commanded to by the cluster software, no more no less, so any error will be a deliberate one programmed in at software level, not as a result of any other spurious factors such as battery voltage.

rebel.ranter · 27-07-2015 8:32pm

In any BMW since the 90s you can code out the speedometer correction factor. It is deliberately set to over read speed to comply with regulations. There is a line in the KOMBI module called BC-Korrecter or something like that.

Cars from the 70s for the most part had a cable driven speedometer, these were only accurate (& set to over read) between 30mph - 65mph as those was the enforceable speed limit area.

Alun · 27-07-2015 9:05pm

Indeed, on my old Focus you could put the instrument cluster in to a kind of debug mode and read the true speed on the digital display. It tallied perfectly with the GPS speed. This proves the 'error' is software based.

OldMrBrennan83 · 27-07-2015 9:07pm

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Del2005 · 28-07-2015 3:35pm

Patww79 wrote: »

This post has been deleted.

GPS doesn't take hills into account so you can be going faster or slower than your true speed.

Innov · 10-10-2021 12:22pm

The odometer is the one responsible for keeping track of the vehicle’s total distance driven throughout its entire lifetime. A speedometer keeps track of the speed at which a car is going at any given time. Finally, a trip meter is similar to an odometer but it only counts the distance of a trip and is resettable.

Hope this helps.. :)

Dial Hard · 10-10-2021 12:26pm

Six years later, I doubt it matters, tbh...

bazz26 · 10-10-2021 12:42pm

This is the second ancient thread dug up today to copy and paste the same thing. Someone has a semi for odometer/speedometer threads.

Odometer v Speedometer

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