Speedo reading high?

[Galaxie390]

Hi Guys,

Recently got my 02 CDT auto tourer, and am over the moon with it.

After driving it for a while I'm getting used to it and compiling a list of things that need addressing.

First query: My speedo is reading too high. I first noticed on hols in France last week.

We timed it over the marked kilometres on the M1 up to Dundalk last night.

We found:
60mph indicated = approx 55mph actual
70mph " " = " 64mph actual
80mph " " = " 75mph actual

Tyres/ wheels: 17", 45 section serpents


Any ideas what's causing this?


Ta,
Rich

[Mike Noc]

'They all do that Sir' :D

They over read - mine does by around 9%

If you have cruise and the message centre you can see that the speedo itself over reads compared with the average speed displayed, and the rest is probably just the safety margin to ensure it can't under read, as that is illegal.

[Roverite]

The law allows a +/- 10% error in the accuracy of a vehicle's speedo. I practice, most manufacturers opt for a + of around 10%. In the 'old' days, this gave an exaggerated speed, which appealed to many car buyers and small boys! Today, it ensures that if you stick to the speed indicated on your speedo, you are unlikely to be booked for speeding.

Although a 10% error is allowed, it still doesn't allow you to exceed the posted speed limit, which is still illegal, however, most Police forces will not prosecute for exceeding the limit by 10% or less, but don't bank on it.

Roverite.

[VMax1000]

The law allows a +/- 10% error in the accuracy of a vehicle's speedo. I practice, most manufacturers opt for a + of around 10%. In the 'old' days, this gave an exaggerated speed, which appealed to many car buyers and small boys! Today, it ensures that if you stick to the speed indicated on your speedo, you are unlikely to be booked for speeding.

Although a 10% error is allowed, it still doesn't allow you to exceed the posted speed limit, which is still illegal, however, most Police forces will not prosecute for exceeding the limit by 10% or less, but don't bank on it.

Roverite.

There is no allowance to under read the speed. The car must always be doing the speed indicated or less.

Laid down by UNECE:


In many countries the legislated error in speedometer readings is ultimately governed by the United Nations Economic Commission for Europe (UNECE) Regulation 39[3] which covers those aspects of vehicle type approval which relate to speedometers. The main purpose of the UNECE regulations is to facilitate trade in motor vehicles by agreeing uniform type approval standards rather than requiring a vehicle model to undergo different approval processes in each country in which it is to be sold.

European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers [4] [5][6] are similar to the UNECE regulation in that they specify that:
The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.

[VMax1000]

The law allows a +/- 10% error in the accuracy of a vehicle's speedo. I practice, most manufacturers opt for a + of around 10%. In the 'old' days, this gave an exaggerated speed, which appealed to many car buyers and small boys! Today, it ensures that if you stick to the speed indicated on your speedo, you are unlikely to be booked for speeding.

Although a 10% error is allowed, it still doesn't allow you to exceed the posted speed limit, which is still illegal, however, most Police forces will not prosecute for exceeding the limit by 10% or less, but don't bank on it.

Roverite.

There is no allowance to under read the speed. The car must always be doing the speed indicated or less.

Laid down by UNECE:


In many countries the legislated error in speedometer readings is ultimately governed by the United Nations Economic Commission for Europe (UNECE) Regulation 39[3] which covers those aspects of vehicle type approval which relate to speedometers. The main purpose of the UNECE regulations is to facilitate trade in motor vehicles by agreeing uniform type approval standards rather than requiring a vehicle model to undergo different approval processes in each country in which it is to be sold.

European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers [4] [5][6] are similar to the UNECE regulation in that they specify that:
The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.

UK Specific:
United Kingdom

A speedometer showing mph and km/h along with an odometer and a separate "trip" odometer (both showing distance traveled in miles).
The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.[11]

The Motor Vehicles (Approval) Regulations 2001[12] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

[Dorset Bob]

With modern tolerances, it must be possible to make speedometers read more accurately than +10% :shrug:.
However, I think it is in the manufacturers interests to do this, as it makes the vehicle appear to be 10% more economical and also increases the main dealer revenue, in terms of service costs and parts by 10% ;)

[T-Cut]

Most car manufacturers will stay within the regulations for the minimal cost. While the regulations allow 0 -10% (+2mph) positive error, they have no reason to improve their manufacturing tolerances. It's basically the luck of the draw and all cars will vary within tolerance. Mine is almost bang on according to my satnav warnings.

[stevestrat]

Have a few of those radar speed indicators around here, you know the ones, show your speed. To get 30mph showing on the speed indicator I need my speedo showing 33-34mph.

[philjudo]

Hi Guys,

Recently got my 02 CDT auto tourer, and am over the moon with it.

After driving it for a while I'm getting used to it and compiling a list of things that need addressing.

First query: My speedo is reading too high. I first noticed on hols in France last week.

We timed it over the marked kilometres on the M1 up to Dundalk last night.

We found:
60mph indicated = approx 55mph actual
70mph " " = " 64mph actual
80mph " " = " 75mph actual

Tyres/ wheels: 17", 45 section serpents


Any ideas what's causing this?


Ta,
Rich
Reading is not to bad with 18" wheels. I notice the difference more on 15" winter wheels I use in the winter.

[T-Cut]

Reading is not to bad with 18" wheels. I notice the difference more on 15" winter wheels I use in the winter.

That's odd, any idea why? Are the tyres much different in tread diameter?

TC

[egremont]

I submitted an item in early 2010 about this subject, Electronic Kit to Correct Speedometer Reading.

Sourced from Jaycar Electronics, it intercepts the blips from the front RHS hub to the IPK. The rate of pulses are changed to reflect the cars adjusted calculations.
Think I placed my report in the Subscriber section. Still giving trouble free service, and can be adjusted if you change your tyre dimensions. This kit only works when the error is linear, in my case the error was 10%., so a correction of 10% removed the error.
egremont :}

[Simon.h]

This is way all police cars had calibrated speedos as all cars over read, just look at your speed on a sat nav its allways lower that your speedo.

[Simon.h]

I submitted an item in early 2010 about this subject, Electronic Kit to Correct Speedometer Reading.

Sourced from Jaycar Electronics, it intercepts the blips from the front RHS hub to the IPK. The rate of pulses are changed to reflect the cars adjusted calculations.
Think I placed my report in the Subscriber section. Still giving trouble free service, and can be adjusted if you change your tyre dimensions. This kit only works when the error is linear, in my case the error was 10%., so a correction of 10% removed the error.
egremont :}

Is it something like this-
http://www.ebay.co.uk/itm/Speedo-Tuner-Speedometer-Corrector-HO-002-/310717486679?pt=UK_Diagnostic_Tools_Equipment&hash=item4858349657

[Trevor1975]

I find my speedometer needle reads about 6-7% optimistic, but the IPK digital readout in option 7.3 reads correctly (in KPH).

If the 'blips' are altered, the needle will read correctly but it will alter a lot of other IPK features such as fuel consumption and give an incorrect odometer reading, I would have thought.... :shrug:

[T-Cut]

If the 'blips' are altered, the needle will read correctly but it will alter a lot of other IPK features such as fuel consumption and give an incorrect odometer reading, I would have thought.

The primary data source for all speed and distance related figures is the signal from the offside ABS sensor. There is no other speed related source. So, if the ABS pulse frequency is modified electronically so that the speedo is exact, then it follows that all other IPK data are also corrected to a similar degree. The odometer will be corrected as will the fuel consumption calculations. However, while the distance/speed related data will become more accurate, the consumption data will always depend on the prevailing driving conditions.

TC

[Roverite]

I think that the difference between the 15" Winter tyres and the 18", is caused by a slight variance in the 'rolling' diameter, probably due to the tyre 'section' being used. Manufacturers get the diameter/circumference equal on different sized wheels by adjusting the cross section height of the tyres fitted. Large wheels are given lower profile tyres and smaller wheels are given deeper profile tyres. Possibly, the Winter tyres are not quite correct. Even worn tyres will give a higher speedo reading than the equivalent new tyre.

I stand corrected regarding the -10%; I just remember the tolerance as being 10% and wrongly assumed that it was +/-. Sorry about that.

Regarding Sat-Nav readings, I am not sure they are perfectly accurate. I believe they measure from point A to point B on the Earth's surface and therefore don't take into account rise and fall, or bends. The distance from the top of one hill to the top of another hill, doesn't take into account the extra distance of going down one side and up the other.

Roverite.

[revor64]

I submitted an item in early 2010 about this subject, Electronic Kit to Correct Speedometer Reading.

Sourced from Jaycar Electronics, it intercepts the blips from the front RHS hub to the IPK. The rate of pulses are changed to reflect the cars adjusted calculations.
Think I placed my report in the Subscriber section. Still giving trouble free service, and can be adjusted if you change your tyre dimensions. This kit only works when the error is linear, in my case the error was 10%., so a correction of 10% removed the error.
egremont :}


I use this, recommended by egremont. I am very pleased with it. I had to correct 9%. It is easy to build, but you have to build it yourself.

http://www.jaycar.com.au/productView.asp?ID=KC5435

[Avulon]

Talkiing of the difference between the speedo and gps speed, and modifying the pulse rate from the hub, the one observation I made was that although the speedo was up to a full 10% out, the vehicle speed from the diagnostics port (using an elm obd bluetooth adamptor) was so close to gps speed as to be negligible, in fact the greatest difference occurred only when accelerating or breaking as the gps lags behind the true speed (it's only really accurate when maintaining a steady speed).

The point is that if you calibrate the speedo by changing the hub pulse frequency to read correctly then, as has been said, everything else will be affected, possibly for the worse. I would be interested to find out if the speedo itself can be calibrated by adjusting the IPK programming (without affecting anything else); that's a question for the T4 guys.

[Trevor1975]

The primary data source for all speed and distance related figures is the signal from the offside ABS sensor. There is no other speed related source. So, if the ABS pulse frequency is modified electronically so that the speedo is exact, then it follows that all other IPK data are also corrected to a similar degree. The odometer will be corrected as will the fuel consumption calculations. However, while the distance/speed related data will become more accurate, the consumption data will always depend on the prevailing driving conditions.

TC

You have missed my point T-Cut. The pulses are being read by the digital IPK correctly. It is just the speedometer needle that is optimistic. with the IPK reading correctly at 110 kph, the speedometer needle is just under 120kph. in fact it varies by a couple of percent day to day....

I find on my car that with the needle saying 74 mph, the actual speed is 70 and the IPK agrees.

Interestingly the digital IPK RPM (7.2?) is very very close to the tacho needle reading.

[Galaxie390]

Hi Guys,

Thank you all for your feedback. If it's 'standard', then at least I know.


Cheers,
Rich