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db

Original Poster:

571 posts

55 months

[news] 
Wednesday 26th January 2011 quote quote all
a pal has broken her shiny saab, speedo is stuck registering 50mph. the quote to fix it is £750 eek
could she use sat nav instead of spending all that cash?
if not, why not?

jaibk

110 posts

58 months

[news] 
Wednesday 26th January 2011 quote quote all
does this help?

Regulation 35 Road Vehicles (Construction and Use) Regulations 1986 states that every motor vehicle shall be fitted with a speedometer except:

a vehicle having a maximum speed not exceeding 25 m.p.h.,
a vehicle which, at all times, is unlawful to drive at more than 25 m.p.h.,
an agricultural motor vehicle driven at not more than 20 m.p.h.,
a motor cycle not exceeding 100cc first used before 1st April 1984,
an invalid carriage first used before 1st April 1984,
a works truck first used before 1st April 1984,
any vehicle first used before 1st October 1937,
a vehicle fitted with an approved tachograph which is required or not.

Vehicles first used on or after 1st April 1984 the speedometer should be capable of indicating the speed in miles per hour and kilometres per hour. Vehicles may instead comply with EC Regulation (Community Directive) 97/39 or ECE Reg 39.
These directives stipulate the markings, graduations of the speedometer and refer to 75/443/EEC which specifies the tolerances.

The indicated speed must never be less than the true speed (it must read exact or high) and between 40km/h and 120km/h the error must not exceed 10% + 2.5 m.p.h. high (true speed/10 + 4kph).
This means at a true speed of 25mph or 40km/h the speedometer may read 40/10+4 = 8km/h or 5mph high = 30mph indicated.

Maintenance

Regulation 36 Road Vehicles (Construction and Use) Regulations 1986 states that the speedometer fitted to a vehicle must be kept free from any obstruction which may prevent it from being easily read and shall at all times it is used on a road be maintained in good working order except if:

the speedometer became defective during the journey being undertaken, or
steps have been taken to have the defect remedied by replacement or repair with all reasonable expedition, or
the vehicle is fitted with an approved tachograph which is required to be fitted under the Community Recording Equipment Regulation (offence is under that regulation).

best i could find

db

Original Poster:

571 posts

55 months

[news] 
Thursday 27th January 2011 quote quote all
that explains the legal side, thanks.
any ideas why a more accurate system isn't legal? no +/- % nonsense

CarbonXKR

1,008 posts

108 months

[news] 
Thursday 27th January 2011 quote quote all
The GPS will not be as accurate as the speedo in detecting instant decrease/increase in speeds due to built in smoothing in the GPS unit. GPS compasses were developed for the marine industry and had to be equipped with two aerials for the set to agree on a heading and subsequent course changes.
Top of the range built in sat-navs as well as having GPS input, are also fed with gyroscopic information and a speedo sensor.

Tunku

7,035 posts

114 months

[news] 
Thursday 27th January 2011 quote quote all
db said:
that explains the legal side, thanks.
any ideas why a more accurate system isn't legal? no +/- % nonsense
It does not explain the fact that a speedo is not required for passing an MOT. I had a Volvo740 that had a defunct speedo and tachometer for the last 100,000 miles of it's life. Never failed to pass the 3 MOTs it had due when in that condition. I used the rev counter to gauge speed.
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davepoth

23,321 posts

85 months

[news] 
Thursday 27th January 2011 quote quote all
It'll MOT fine, but it's a C&U violation that could be an offence if the police ever spotted it because it's against the rules to have a broken speedometer. You would need to gauge the possibility of being caught for it against the price of fixing it. Being caught would be extremely, extremely unlikely. In fact the only way I can imagine it happening is if your friend admitted it to a police officer.

jondude

1,535 posts

103 months

[news] 
Thursday 27th January 2011 quote quote all
Only other plan B I can think of is to visit a breaker's yard or, and I did this a few times when a biker as ridiculous as it sounds - buy a speedo unit with a car wrapped around it!

Honestly - was quoted 150 quid for a fuel tap for my Honda, over 500 quid for an exhaust system.....bought the same bike with a blown engine(but working tap and decent full exhaust system) for 50 quid......

streaky

19,311 posts

135 months

[news] 
Thursday 27th January 2011 quote quote all
db said:
any ideas why a more accurate system isn't legal? no +/- % nonsense
One is: "The indicated speed must never be less than the true speed (it must read exact or high)". The 'read high' tolerance is so that there can be no defence of the speedometer displaying a speed lower than that at which the vehicle is travelling. Any increase in the circumference of a tyre will require appropriate changes to the speedometer, a reduction (other than normal wear) will require the speedometer to be changed if the speed displayed is outside the margin.

Streaky

Puff the magic..

584 posts

66 months

[news] 
Thursday 27th January 2011 quote quote all
db said:
that explains the legal side, thanks.
any ideas why a more accurate system isn't legal? no +/- % nonsense
What system is that then? Because a GPS system will not indicate the vehicle speed adequately unless you get a very expensive one? £300 GPS from Halfords or Argos won't do the job.

vescaegg

3,869 posts

53 months

[news] 
Thursday 27th January 2011 quote quote all
If you are anywhere near London, I used cluster repairs uk (I believe they were off the north circular) when my BMW was doing the same. The basically cleaned and recalibrated the dials. Cost about £60 I think and never had a problem since.

E Ponym

1,179 posts

153 months

[news] 
Thursday 27th January 2011 quote quote all
db said:
that explains the legal side, thanks.
any ideas why a more accurate system isn't legal? no +/- % nonsense
GPS is much more accurate than a conventional speedo. GPS measures the speed on the road whereas most speedos measure based on wheel rotation and do not take any account of the amount of tread on the tyre - which can make a significant difference.

The OP's friend ought to find a way to get the SAAB speedo to read zero and then use a GPS unit. How will anybody be able to check on a roadside check that the speedo is working - jack the wheels up?

Is the odometer working? This could possibly cause issues when the car is sold because successive MOT's will show the same mileage.

streaky

19,311 posts

135 months

[news] 
Thursday 27th January 2011 quote quote all
E Ponym said:
db said:
that explains the legal side, thanks.
any ideas why a more accurate system isn't legal? no +/- % nonsense
GPS is much more accurate than a conventional speedo.
Not at any moment in time, as CarbonXKRexplained above; only over a distance.

Streaky

walm

4,742 posts

88 months

[news] 
Thursday 27th January 2011 quote quote all
streaky said:
E Ponym said:
db said:
that explains the legal side, thanks.
any ideas why a more accurate system isn't legal? no +/- % nonsense
GPS is much more accurate than a conventional speedo.
Not at any moment in time, as CarbonXKR explained above; only over a distance.

Streaky
Yeah, those pesky worn down tyres, if only they stopped losing satellite coverage in built up areas.
Then I would know my speed...

defblade

3,314 posts

99 months

[news] 
Thursday 27th January 2011 quote quote all
streaky said:
E Ponym said:
db said:
that explains the legal side, thanks.
any ideas why a more accurate system isn't legal? no +/- % nonsense
GPS is much more accurate than a conventional speedo.
Not at any moment in time, as CarbonXKRexplained above; only over a distance.

Streaky
Only measures horizontal speed, too, so it'll be wrong going up and down hills. And because of the over-a-distance thing, the reading always lags (most obviously showing 5 or 10mph for a few seconds when you've actually stopped). And they're not made or tested to any particular standards, either....

....I'm really not sure where this "they're more accurate" thing comes from in the first place.....

db

Original Poster:

571 posts

55 months

[news] 
Thursday 27th January 2011 quote quote all
davepoth said:
In fact the only way I can imagine it happening is if your friend admitted it to a police officer.
ahh, she works as a civilian employee of a scottish police force and has undoubtedly voiced objections to silly money.

the needle is stuck at 50mph so easily noticed when stationary.

thanks for all info regarding gps and it's limits, feeling slightly more educated now smile

aw51 121565

4,030 posts

119 months

[news] 
Friday 28th January 2011 quote quote all
defblade said:
streaky said:
E Ponym said:
db said:
that explains the legal side, thanks.
any ideas why a more accurate system isn't legal? no +/- % nonsense
GPS is much more accurate than a conventional speedo.
Not at any moment in time, as CarbonXKRexplained above; only over a distance.

Streaky
Only measures horizontal speed, too, so it'll be wrong going up and down hills. And because of the over-a-distance thing, the reading always lags (most obviously showing 5 or 10mph for a few seconds when you've actually stopped). And they're not made or tested to any particular standards, either....

....I'm really not sure where this "they're more accurate" thing comes from in the first place.....
My bold. But how inaccurate? GPS works off several satellites scattered around the sky, so this hill-climbing speed/distance error largely cancels itself out, I believe; it's not a significant error.

Put a GPS system in the car, and measure the distance between emergency phones on a motorway - or even better, get your passenger to reset the tripmeter and GPS when you pass one motorway emergency phone and check the readings at the tenth emergency phone you pass after the initial one (the first one you pass after about a minute is "number one", the next one passed after two minutes is "number two" and so on) - and compare the figures at motorway emergency phone "number ten".

The GPS will say 10.0 miles, and the tripmeter will probably say 11ish... The true distance driven in your experiment, with a few exceptions, is 10.0 miles 'cos the emergency phones of doom are usually a mile apart wink .

defblade

3,314 posts

99 months

[news] 
Friday 28th January 2011 quote quote all
aw51 121565 said:
The GPS will say 10.0 miles, and the tripmeter will probably say 11ish... The true distance driven in your experiment, with a few exceptions, is 10.0 miles 'cos the emergency phones of doom are usually a mile apart wink .
Where they measured out by GPS or odometer in the first place? wink
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