Speedos and other things

[Lenny HB]

Here is the legal requirement:-

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.[14]

The Motor Vehicles (Approval) Regulations 2001[15] 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.

 

[Jaws]

You are absolutely correct about Speedos recording higher than reality.
When I was a surveillance motorcyclist chasing a 911 North up the M1 on a ZZR1100 my Speedo was reading 155 but apparently, according to Herts Police in their Cosworth, I was actually doing only 144.
(Allegedly)

Got a certificate ( posh word for a scruffy bit of paper with a print out on ) from an event organised by Performance Bikes on an ex USAF air base.. 184.6mph. Speedo was showing over 200 mph

 

[pappajohn]

When there's a really loud bang it can only be one of three things, or a combination of all three.
1 a blow out
2 engine blown up
3 sound barrier broken.

In any event your speedo accuracy isn't an issue.

 

[Lenny HB]

From the legal bit I posted eairler if you are doing a genuine 70 mph your speedo can read anyting from 70 mph to 83.25 mph and it will be legal and within tolerance.

 

[pappajohn]

From the legal bit I posted eairler if you are doing a genuine 70 mph your speedo can read anyting from 70 mph to 83.25 mph and it will be legal and within tolerance.

But how do you know just how inaccurate your speedo is.? It can show 83.25 IF it's 10% plus 6.25 mph fast.
If its only 5% fast you were speeding by 3.25mph......

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[Lenny HB]

But how do you know just how inaccurate your speedo is.? It can show 83.25 IF it's 10% plus 6.25 mph fast.
If its only 5% fast you were speeding by 3.25mph......

???
If it's 5% fast @ 70 it would read 73.5.
Your turn to be brain dead.

 

[tonyidle]

But that's a mechanical mismatch.
Had the correct sender been fitted it would have been much closer.

I'm only assuming that was the case - I can't see any other explanation of such an error. Unless others have experienced similar?

Isn’t rubber stretchy then ?

If it was only rubber then Yes. But tyres have cords to prevent stretching getting out of hand. Tread stretch doesn't generally occur to any meaningful extent by increasing the tyre pressure (within limits) but does occur to some degree caused by speed and centrifugal (centripetal) forces. This can be seen in a dramatic fashion on drag car rears that are designed to do just that.

 

[tonyidle]

But odometer aren't

True - and the odometer may be no more accurate than the speed display.

 

[tonyidle]

Put enough air in and the tyres end profile will be more like a motorbike tyre.... Sidewalls pull in, tread pushes out until the two are balanced then, if it hasn't already, it goes bang. .
Think top fuel dragster rear tyres, they expand by a foot or more.

"Enough air" would have to be considerably more than "enough air".

 

[tonyidle]

As an aside it is regularly recommended that if tyre / wheel size is to be increased then a lower profile tyre should be used to maintain speedo accuracy. Before choosing a tyre profile it would be very wise to ascertain existing speedo error then calculate the correction offered by various profiles. In most cases increasing diameter and retaining the existing profile improves speedo accuracy whilst still staying on the right side of the law.

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[Lenny HB]

True - and the odometer may be no more accurate than the speed display.

Usually is, last van speedo read over 10% high, cheked the odometer over a few hundred miles almost spot on.

Also with the speedo it's not just about where the needle is, how accurate are the dial markings. The odometer is just a digital readout so no marking errors.
On my current van when I put the kpm dial back in it is noticably more accurate than the mph one.

 

[tonyidle]

Usually is, last van speedo read over 10% high, cheked the odometer over a few hundred miles almost spot on.

Also with the speedo it's not just about where the needle is, how accurate are the dial markings. The odometer is just a digital readout so no marking errors.
On my current van when I put the kpm dial back in it is noticably more accurate than the mph one.

I said 'may be' because it isn't something I've ever checked so good to know. It no doubt helps too that modern speedos have no physical connection between the speedo & odo.

 

[Tony50]

I don't trust any speed recorder , as I frequently go over the Orwell Bridge on the A14 with Average speed cameras I never go over dead on speed limit I always travel a mile or two slower, since when they installed cameras in 2016 in the first 3 months they caught over 4000 drivers !! and are still catching loads of drivers , so beware if you type in Orwell bridge Ipswich on your computer you can see the results yourself

 

[JoCo]

Most manufactures now build in a 6% over reading on speedos. so they dont face lawsuits.
Most times your GPS will read 100 KPH and the speedo will be 105 - 106 KPH

 

[Lenny HB]

Most manufactures now build in a 6% over reading on speedos. so they dont face lawsuits.
Most times your GPS will read 100 KPH and the speedo will be 105 - 106 KPH

See my post #31

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[JaG]

Just a point - SatNav's are not always right either. I proved that because I have a TomTom and an RAC 05 Dashcam that also records speed, average speed, location, etc. The RAC 05 always reads 3mph faster than the SatNav, and the SatNav reads approximately 2mph faster than the speedometer.

 

[GWAYGWAY]

I have a gps speedo on the dash to give me the choice of kph and mph , it reads 2 mph lower than both the speedo and the gps Tom Tom which are the same, who is right? The Tom Tom and speedo or the cheap chinese thing, I actually think China wins.

 

[G-RMPS]

I know a chap that claims the police came to see him in his mot garage after pacing his ferrari in their helicopter, he claims that they told him to slow down in future as the speedo ? In the helicopter was reading 160mph but wasn’t acceptable in court as it wasn’t calibrated.
I have no problem believing he was doing 160
He is pretty wild.

Most helicopters won’t do 160mph, and their “speedo” displays air speed - so speed over the ground depends on wind strength and direction at the height the helo is flying. If true, the ground speed of the ferrari is being reported from a gps not the plane’s speedo.

 

[Lenny HB]

Just a point - SatNav's are not always right either. I proved that because I have a TomTom and an RAC 05 Dashcam that also records speed, average speed, location, etc. The RAC 05 always reads 3mph faster than the SatNav, and the SatNav reads approximately 2mph faster than the speedometer.

GPS speed will only be truly accuract if the road is dead straight and at a constant altitude but they are still probably the most accuract option we have, unless you fit a calibrated speedo.

 

[gus-lopez]

I know a chap that claims the police came to see him in his mot garage after pacing his ferrari in their helicopter, he claims that they told him to slow down in future as the speedo ? In the helicopter was reading 160mph but wasn’t acceptable in court as it wasn’t calibrated.
I have no problem believing he was doing 160
He is pretty wild.

Trafico here have 3 helicopters fitted with 'pegasus' speed monitoring which requires no additional calibration.I believe it triangulates via gps & photographs & records from its fixed position. It also appears that once in position it is actually 'flown' by the computers to maintain height/position. On the occasions you can actually see one, due usually only to the sun glinting off of it, they are fascinating as they are absolutely motionless, just like a dot painted on a picture.

Easily do-able using motorway mile markers and a stopwatch.

I usually check my speed using them as they are kilometre indicators with 100m markers. Always fascinated me why the UK use MPH & yesat all the road distance markings are in kilometres ?

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[Lenny HB]

I usually check my speed using them as they are kilometre indicators with 100m markers. Always fascinated me why the UK use MPH & yesat all the road distance markings are in kilometres ?

I always wondered why the road marking never made sence, that could explain it.

 

[Feltwell]

From the title of this thread I half thought it was going to be about the bizarre French insistence on men wearing "budgie smugglers" at all their swimming pools...…...

 

[john lillie]

according to my Garmin, my Fiat based Hymer reads well over 10% optimistic, whilst my Honda CRV is within 1mph of the satnav at any speed. Confirms to me what I always thought about Honda (I have CRV, 3 Honda bikes and Honda lawnmower!) if only they made a commercial vehicle that could be used in motorhomes.....

 

[Lenny HB]

according to my Garmin, my Fiat based Hymer reads well over 10% optimistic, whilst my Honda CRV is within 1mph of the satnav at any speed. Confirms to me what I always thought about Honda (I have CRV, 3 Honda bikes and Honda lawnmower!) if only they made a commercial vehicle that could be used in motorhomes.....

They do but they are not imported to Europe.

 

[RioRob]

From my experience it doesn't seem to matter what speedos indicate as a large proportion of drivers never look at them or ignore them. Otherwise why is there such a problem with speeding!

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[autorouter]

Incidentally motorway markers are at 100 metre spacing.

In the UK they are every tenth of a mile (176 yards).

I have vague memories of markings in furlongs (eighths of a mile) and measuring in chainage for tarmac laying (10 chains = 1 furlong). No doubt they'll be back after March next year.

 

[autorouter]

GPS speed will only be truly accuract if the road is dead straight and at a constant altitude but they are still probably the most accuract option we have, unless you fit a calibrated speedo.

I think the most accurate method is doing a constant speed as seen on the speedometer, using mile markers and a stopwatch to calculate actual speed.

 

[john lillie]

"mile markers" on motorways are definitely in KM, not Miles! The marker near LFE services in Leicestershire shows approx 160, which is about 100 miles from the start, wherever that may be, and they go up in tenths, i.e. 160.1, 160.2 etc up to 160.9, then 161

 

[Jonno1103]

I don't know if they still do but many taxi drivers replaced their wheels with smaller ones so the metre ticked over quicker.

 

[obanvanman]

M1, Junction 5 on slip southbound.
5 Series BMW in the outside lane cruising at about 70.
We joined the motorway and pulled it over about a mile down the road.
The driver gets out and starts going berserk at me.
^No way, no f*****g way. I saw you on the slip road you f*****g lying twat. There is no way you are doing me for speeding^
^You are absolutely correct sir, I am not doing you for speeding.......
.......he is^ I then pointed up.

Hovering above us was India99, the Met Police helicopter. The Traffic officer on board had a portable PolicePilot on board and had been following the car using distance markers from Hertfordshire.

(It didn't really help matters that the PC was hanging out of the window waving at us)

Aaaaah....Happy days

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