Speedometer Accuracy

[pensioner]

Not sure if this should be here or in Chit Chat but I'm sure it'll be moved or deleted if it's wrong.

I was just curious as to which speedo is likely to be more accurate,( as I remember the days when they were only needed to accurate to within 10% ),,my car which is a Toyota or my Sat Nav; as there is often a difference of up to 2mph. between them.

 

[WoodlandsCamper]

The sat nav will be more accurate and will read lower than your vehicle speedo.

 

[chrisbee1]

It is still the case that speedometers can show up to 10% faster than true speed. They are not allowed to show a slower speed than actual. Sat navs tend to be more accurate but are not 100% .

 

[NigelHutson]

That's correct Chriee1. The vast majority of speedometers read fast (i.e. indicated 30mph is usually in the region of a true 27/28mph).

A sat nav will generally be more accurate, but is dependant upon how many satellites it's receiving. The more the better, and if it's not receiving enough for proper triangulation, it can be inaccurate.

However, if drivers stick with the speedometer readout, they'll never be caught speeding (providing of course the indicated speed is within the speed limit!).

 

[Raywood]

It is a long time since I saw anything on speedo accuracy. That was by Diesel Car who tested a range of cars and every one read over the true speed and I have no reason to believe this has changed.
The depth of tread on your tyres also will affect this to some extent as with about 6 mm of tread worn away by the time you change them you will be reading a speed a bit more over than on new tyres.
My speedo also reads higher than the sat nav usually by two or thee mph at about 60.

 

[Hutch]

Beware, of the M1, junction 11 to 15, Luton / Milton Keynes area, the local Media are reporting that when the advisory speed warning are in place the speed cameras will possibly be set to trigger at 2 mph over the adnvised limit.
Speedo on my Santa Fe registers 5 mph more than the sat nav (snooper) at 60.
Hutch.

 

[Sproket]

This Link is useful for calculating the tyre size and it also gives a difference in what your speedometer would read when deciding to fit different size of tyres than OSF by the manufacturer. You will need to scroll down to the tyre size calculator. ....

 

[Dustydog]

The Sorento speedo compares very well at low speeds with the Tom Tom.
As speed rises the Speedo is 3 or 4 miles faster than the Tom Tom.

 

[BedfordGeorge]

I have checked my SatNav (Snooper) many many times against the warning signs that flash and show your speed ,as you enter restricted zones, and virirtually without exception they show the same speed, whereas the car alway reads faster.

 

[FrintonMike]

I once read that the speed showing on a sat nav is not accurate at the time it shows on the screen due to the time lag between the sat nav and the satellite

 

[ProfJohnL]

Beware, of the M1, junction 11 to 15, Luton / Milton Keynes area, the local Media are reporting that when the advisory speed warning are in place the speed cameras will possibly be set to trigger at 2 mph over the adnvised limit.
Speedo on my Santa Fe registers 5 mph more than the sat nav (snooper) at 60.
Hutch.

Hello Hutch,
There is no such thing as a legal tolerance for speeding. The clue is in the name "speed limit" where limit represents the accumulation of all measurements and tolerances. The.oft quoted "tolerance" is the practical result of of the Association of Chief Police Officers (ACPO) offering guidance to forces on how to improve prosecution results for speeding. By ignoring small infringements which have a higher probability of being successfully challenged on the basis of uncertainty, this leaves prosecutions of moderate to severe speeding which are far less likely to fail.

There was a recent article where more recent technology could be shown to reliably detect and record speeds more accurately, which would lead to a higher prosecution success rate for even minor infringements of speed limits. And there have long standing reports of certain areas of the road network where zero tolerance to speeding have been adopted by the local authorities.

It is always the drivers responsibility to not exceed the speed limits, and faulty or inaccurate speedometers are not accepted as a mitigating circumstance. This is one reason why modern vehicle speed meters are not allowed to read slower than the vehicles real speed.

 

[WoodlandsCamper]

I once read that the speed showing on a sat nav is not accurate at the time it shows on the screen due to the time lag between the sat nav and the satellite

It will be correct if you are travelling at a constant speed, eg on a motorway. The unit in your vehicle calculates the speed every few seconds from the time and positional data provided by the satellites. So yes, there will be a slight lag for the calculations to take place, so if your speed is changing rapidly there will be more error on instantaneous speed.

 

[ProfJohnL]

I once read that the speed showing on a sat nav is not accurate at the time it shows on the screen due to the time lag between the sat nav and the satellite

It will be correct if you are travelling at a constant speed, e.g. on a motorway. The unit in your vehicle calculates the speed every few seconds from the time and positional data provided by the satellites. So yes, there will be a slight lag for the calculations to take place, so if your speed is changing rapidly there will be more error on instantaneous speed.

Hello Woodlands and Mike,

Sat Navs work out their position by triangulating their distance from a three or more satellites. They workout their speed by comparing consecutive positional readings and the time taken between the readings. Consequently the time it takes for the signal to travel from the satellite to the sat nav cancel out so the propagation delay has no effect on the speed calculation made by the sat nav.

However the positional accuracy of a sat nav can vary often it can be as good as +/-6M or as poor as +/- 50M ! That can affect the accuracy of the speed calculation for example, if a car is moving at 35mph, (60Kmh) it will be covering 16.6m per second. If the positional accuracy of the sat nav is poor (+/-50M) then in theory the first reading could be under reading by 50M and the second reading could be over reading by 50 a difference of 100m + the movement of the car giving two positional readings 116.6m apart. That equates to a speed of 419kmh or 256mph!!

However sat navs are a bit cleverer than that , and they tend to average consecutive readings, but you would have to confirm with each sat navs manufacturer their averaging period or sample rates, so in practice they will rarely display such outrageous readings.

Sat navs are most accurate when you are moving a straight line at a constant speed with good signals from 4 or more satellites, and will almost certainly by much more accurate than the vehicles own speedometer.

 

[Gagakev]

Bit of Trivia for you, may be relevant may be not......

our local radio station pose a brain teaser question every morning and todays question........

What does the average person do 18,000 times in there life time.............

The anserw.......................................

SPEED hmy:

 

[JonnyG]

"It is always the drivers responsibility to not exceed the speed limits, and faulty or inaccurate speedometers are not accepted as a mitigating circumstance. This is one reason why modern vehicle speed meters are not allowed to read slower than the vehicles real speed.[/quote]"

That is not entirely accurate.. with regards to HGV's "modern vehicle" assuming our various sat navs to be accurate over a given distance . we have 2 new lorries that will do 57 mph slightly more. [92kph] ..bear in mind since 2012 they are supposedly to be govern to 89kph down from 90kph.. the speedo reads 90kph and these speeds are factory set... in the past we have had new lorries supposedly restricted to 90kph [56mph] that did varying speeds from 54 to 58mph..always with the speedo showing 90kph [56mph] ... a search of the internet will reveal a fare few restricted hgv's that will show 90kph [56] but will exceed this speed by 1 or even 2 mph...
and we are not talking silly magnets here,they came out the factory that way... or indeed on the 2 yearly tacho inspection /calibrations they came out faster or slower than they went in [VOSA controlled] ...i know my sat nav is accurate but it is not in any terms reconsidered officially, so my speedo shows 90kph [56] the legal speed but my lorry does 57mph plus.... i await the day where the powers that be hold me responsible for traveling above the 56mph speed limit....in the same way as i await the owners of VW cars still driving them being held accountable for their cars having the wrong emissions

 

[seth1]

100% correct Jonny G.The descrepincy is down to the K factor setting in the tacho head.Speed for speed readings are allowed as far as 4% inacurate.

 

[ProfJohnL]

100% correct Jonny G.The descrepincy is down to the K factor setting in the tacho head.Speed for speed readings are allowed as far as 4% inacurate.

Back in 1958 and over the following decade, a world wide accord was drawn up known as ECE-R39 to which most of the major motor manufacturers through their national governments. Certainly vehicles sold in the EU are expected to comply with its provisions, one of which relates to the accuracy or calibration of vehicle speedometers. I do not have its text, but there is plenty of comment about the inference of speedometers not being permitted to under read speeds, and must be within a a certain percentage of over read.

Whether this applied to HGV I do not know but for private motors its certainly does.

Speedometers that take their source from the vehicles transmission will always demonstrate variability, because the effective diameter of the driven wheels changes over the life of the tyre, and also with changing pressures, either due to load, or temperature.

The same variations in tyre diameter will also affect the vehicle's milometer.

 

[Gafferbill]

........a completely different set of regulations apply to HGVs.

The speedometer fitted to HGVs is part of the vehicles Tachograph recording equipment.
This instrument records the vehicles aspects of operation one of which is speed and another is time of day
Since these records have to be retained for future reference and can be used as evidence in a court of law, the speedometer in an HGV is much more accurate than a private vehicle with its calibration being checked for accuracy every 2 years by law.
The speedometer and sender unit are sealed to prevent tampering and it is an offence to break the seal.

 

[chrisbee1]

A few nearly but not quite right statements about HGV's, but then I'm a pedant. :blink:

Remember it's the tacho and not the speedo which is calibrated. You will get a more accurate indication of your speed if you look at the tacho speed reading.

"bear in mind since 2012 they are supposedly to be govern to 89kph down from 90kph"

I certainly hope this isn't the case. I was setting speed limiters at 90 kph last year before I retired.

"the speedo reads 90kph and these speeds are factory set"

Yes, but can be adjusted by the dealer.

"a search of the internet will reveal a fare few restricted hgv's that will show 90kph [56] but will exceed this speed by 1 or even 2 mph..."

"Speed for speed readings are allowed as far as 4% inaccurate"

In use there is a +/- tolerance of 6 kph for the tacho. No % figure is given. It follows that as the VECU gets it's speed signal from the tacho then the RSL can also be out by this margin. Additionally, the RSL is permitted to exceed the required stabilised speed by 1kph. In other words, an HGV can in theory travel at a stabilised true speed of 97 kph and still be within the allowed tolerances. In reality, anyone who is licensed to calibrate tachos and RSLs and is honest will get it pretty much to 90kph every time.

"the speedometer in an HGV is much more accurate than a private vehicle with its calibration being checked for accuracy every 2 years by law".

It's the tacho, not the speedo, which is checked/calibrated.

"so my speedo shows 90kph [56] the legal speed but my lorry does 57mph plus.... "

The legal speed limit is 60 mph in the UK.

"The speedometer and sender unit are sealed to prevent tampering and it is an offence to break the seal."

Again, it's the tacho, not the speedo, which is sealed. Since the fitting of "cassette" and then digi tachos I've never seen a speedo which is sealed.

It is not an offence for anyone to break the seal, for instance to repair gearbox, loom etc. However, the vehicle must be presented for calibration/resealing before it continues it's normal role.

 

[JonnyG]

A few nearly but not quite right statements about HGV's, but then I'm a pedant. :blink:

Remember it's the tacho and not the speedo which is calibrated. You will get a more accurate indication of your speed if you look at the tacho speed reading.

"bear in mind since 2012 they are supposedly to be govern to 89kph down from 90kph"

I certainly hope this isn't the case. I was setting speed limiters at 90 kph last year before I retired.

"the speedo reads 90kph and these speeds are factory set"

Yes, but can be adjusted by the dealer.

"a search of the internet will reveal a fare few restricted hgv's that will show 90kph [56] but will exceed this speed by 1 or even 2 mph..."

"Speed for speed readings are allowed as far as 4% inaccurate"

In use there is a +/- tolerance of 6 kph for the tacho. No % figure is given. It follows that as the VECU gets it's speed signal from the tacho then the RSL can also be out by this margin. Additionally, the RSL is permitted to exceed the required stabilised speed by 1kph. In other words, an HGV can in theory travel at a stabilised true speed of 97 kph and still be within the allowed tolerances. In reality, anyone who is licensed to calibrate tachos and RSLs and is honest will get it pretty much to 90kph every time.

"the speedometer in an HGV is much more accurate than a private vehicle with its calibration being checked for accuracy every 2 years by law".

It's the tacho, not the speedo, which is checked/calibrated.

"so my speedo shows 90kph [56] the legal speed but my lorry does 57mph plus.... "

The legal speed limit is 60 mph in the UK.

"The speedometer and sender unit are sealed to prevent tampering and it is an offence to break the seal."

Again, it's the tacho, not the speedo, which is sealed. Since the fitting of "cassette" and then digi tachos I've never seen a speedo which is sealed.

It is not an offence for anyone to break the seal, for instance to repair gearbox, loom etc. However, the vehicle must be presented for calibration/resealing before it continues it's normal role.

the 2012 quote does worry me somewhat. it reminds me of a conversation i had with one of our drivers in 2014. who stated HGV's were restricted to 89 kph.. i of course argued the piont as always that it was actually 90 kph. he then pointed out that our new lorries Renault,Man, Mercedes all had stickers in their windscreen with small stickers stated 89kph.. i did a google search at the time and it was a long search. and low and behold after some searching came across a EU directive stating that as of 2012 all hgv's above 12 tonne to be restricted to 89kph. now maybe as it is only a directive it may not be law. and i believe it had something to do with emissions /mpg. although frankly i think its a first step to harmonising HGV speeds across the EU to 80 kph. alas at the time it took me some searching to find and a quick search today has revealed nothing but i will take a deeper look once i have sufficient time at work.. my concern now is these new lorries are definitely coming out of the factory restricted to 89kph with stickers attached as i stated it is a EU directive otherwise why would manufacturers go to the trouble to restrict them buy a further 1 kph?. now if those in the trade are uninformed about such directives,and haven't wondered why newer lorries have 89 kph stickers attached then when these vehicles come in for there 2 yearly inspection/calibration they are going to be reset to 90 kph!
instead of 89kph. if one assumes this directive is solely based on fuel consumption and the lowering of emissions then clearly it fails because not everyone it appears that should know, does.

 

[chrisbee1]

When the 2 year calibration takes place the tacho is invariably programmed with a new "K" factor. Now, to keep the RSL accurate that figure should also be programmed into the vehicles ECUs. Unfortunately only main dealers are likely to have the equipment to enable this to be done. It's one of the downfalls of the legislation. There is no requirement to re-programme the rsl when the tacho is calibrated. As a company we would do it on the make of truck we were franchised for but of course other calibration centres didn't. Because the programming required connection to a system in Sweden there was a cost involved meaning we lost work to our non franchised competitors who of course couldn't keep the RSL accurate but charged less. Maybe the manufactures are taking this into account so deliberately building in a 1kph safety net? Who knows?

 

[Dustydog]

:woohoo: :woohoo: :woohoo: hmy: hmy:
Just got summonsed for speeding ( solo) down in North Devon. 75 on a 60 road . I could have sworn I never went over 70 but no point in arguing. My fault as I was overtaking a coach on am overtaking lane.
Oh well first offence since 1996. Hope it's just a fine and 3 points

 

[WoodlandsCamper]

Hope it's just a fine and 3 points

Or an expensive speed awareness course? :evil:

 

[SheffSean]

I recently bought a TomTom go 6000 and wondered if it was at fault, it's brand new so it looks like my speedo is out by 2 mph too.

Sean.

 

[Sproket]

:woohoo: :woohoo: :woohoo: hmy: hmy:
Just got summonsed for speeding ( solo) down in North Devon. 75 on a 60 road . I could have sworn I never went over 70 but no point in arguing. My fault as I was overtaking a coach on am overtaking lane.
Oh well first offence since 1996. Hope it's just a fine and 3 points

Hmm !