Stated maximum speed -safe maximum or absolute maximum?

[jon0844]

Thats quite a large discrepancy in this case - a true 79 mph equating to an indicated 95. In my car 70 mph on my GPS is about 74 mph on the speedometer. Car speedos are not allowed to under read by law so i guess someone doing an indicated 100 could only be doing a true 90 mph. But the train speedometer is accurate to within a few mph. I did a check when I was a conductor on a few units.

On a modern car, you'll find that the ECU probably offsets the speed reading by 3-4mph to ensure there's no chance of under reading. In my experience, this doesn't seem to change at lower or higher speeds.

On some cars, you can unofficially adjust this or enable a 'service' mode that will show a more accurate reading - which could potentially break the law. Indeed, if you changed your wheels/tyres then you might need to get a dealer to enter the new settings to make sure the speedo isn't out by more.

A GPS is of course the best way to see what speed you're really doing. I usually refer to this on motorways, and it does mean being able to go a little faster when you see a police car (where the speedo will be regularly calibrated) or are driving through 50mph roadworks!

 

[notadriver]

In theory it's a good idea. In practice in busy roadworks everyone is travelling at an indicated 50 or slower. I have a user selectable speed limiter (up to 99 kph) on my coach so ill set that at 50 mph in roadworks and use the cruise control at 62 mph once clear

 

[142056]

On a modern car, you'll find that the ECU probably offsets the speed reading by 3-4mph to ensure there's no chance of under reading. In my experience, this doesn't seem to change at lower or higher speeds.

You're right, and this was most useful for my driving instructor in his Vauxhall Corsa, with a digital speedometer below the clock in the centre. Words were had along the lines of "You can see your speedometer, I see this one and you are doing 31, so you must definitely be over on yours..." Whoops....

 

[cjmillsnun]

Although you do occasionally see cars doing >100mph, it is quite rare as at that speed drivers are risking an instant ban if caught.

Also, almost every car speedometer over-reads. For long journeys we use an GPS speed app, which sounds an alarm at 79mph (70 + 10% + 2mph) and at that point the car's speedo is reading around 95. Other drivers do overtake at this speed, but that doesn't mean they're doing >100

As far as cars doing over 100MPH I get overtaken by them every single day on the A3.

GPS itself isn't as accurate as you think unless you spend thousands on surveying grade kit, and car speedos must be within 10% of the car's speed in an over read direction only for type approval.

 

[asylumxl]

I have been overtaken by cars on the A505 between baldock and Royston before when doing 90 in a train! And sections of that road are single carriage way to I believe.

To be honest, the A505 was the road I was originally thinking of lol. Not quite at Baldock, but not far from it!

 

[asylumxl]

GPS itself isn't as accurate as you think unless you spend thousands on surveying grade kit

Even then it won't be that reliable. Without corrections from various OS stations it's as bad as any other GPS. With corrections it's fine when stationary, but useless when on the move or when near any kind or size of tree...

 

[507 001]

Thats quite a large discrepancy in this case - a true 79 mph equating to an indicated 95. In my car 70 mph on my GPS is about 74 mph on the speedometer. Car speedos are not allowed to under read by law so i guess someone doing an indicated 100 could only be doing a true 90 mph. But the train speedometer is accurate to within a few mph. I did a check when I was a conductor on a few units.

On a modern car, you'll find that the ECU probably offsets the speed reading by 3-4mph to ensure there's no chance of under reading. In my experience, this doesn't seem to change at lower or higher speeds.

On some cars, you can unofficially adjust this or enable a 'service' mode that will show a more accurate reading - which could potentially break the law. Indeed, if you changed your wheels/tyres then you might need to get a dealer to enter the new settings to make sure the speedo isn't out by more.

On my focus I can access a diagnostic mode which brings up a digital speedo which is more accurate than the standard clock.

I would imagine under/over inflated tyres could have a marked effect on speedo accuracy

 

[cjmillsnun]

Even then it won't be that reliable. Without corrections from various OS stations it's as bad as any other GPS. With corrections it's fine when stationary, but useless when on the move or when near any kind or size of tree...

agreed, which backs up my point even more that the cheap grade GPS systems in a sat nav shouldn't be used as an accurate speed measurement device. Because they aren't.

 

[notadriver]

I've tested 3 GPS devices simulataneously at a steady speed in ideal conditions. All were within 1 mph of each other. That's accurate enough for me.

 

[cjmillsnun]

I've tested 3 GPS devices simulataneously at a steady speed in ideal conditions. All were within 1 mph of each other. That's accurate enough for me.

I'd rather stick to the car's speedo. I know I'm under the speed limit then.

 

[asylumxl]

I've tested 3 GPS devices simulataneously at a steady speed in ideal conditions. All were within 1 mph of each other. That's accurate enough for me.

They were I assume, using the same satellites in the same location. Couple this with the fact they may well share internal components and that most consumer GPS uses Kalman filtering, it's not really surprising.

At the end of the day it's actually an estimate.

 

[lyndhurst25]

Very interesting. Given a completely clear autoroute from Calais to Paris surely the most hardened car fanatic must admit that the Ferrari wouldn't be able to keep up with the Eurostar all the way to Paris?

Sounds an idea for a TV programme challenge. And on that bombshell...

Don't trains all have data-recorders now that constantly log the speed? Back in the 1970s and 80s you would sometimes hear reports of Deltics running 110mph and HSTs above 130mph. Perhaps drivers are more reluctant to speed now for fear of being caught.

 

[61653 HTAFC]

Don't trains all have data-recorders now that constantly log the speed? Back in the 1970s and 80s you would sometimes hear reports of Deltics running 110mph and HSTs above 130mph. Perhaps drivers are more reluctant to speed now for fear of being caught.

Pre OTMR, there were often stories of 110s, even 141s being pushed towards 90mph. Of course a fair bit of this might have been 'fisherman's tales' but I imagine no driver worth his or her salt would make such boasts now, whether they were true or not!

 

[A-driver]

Pre OTMR, there were often stories of 110s, even 141s being pushed towards 90mph. Of course a fair bit of this might have been 'fisherman's tales' but I imagine no driver worth his or her salt would make such boasts now, whether they were true or not!

When they ran slam doors on the fenchurch street route it is said that there was a set of empties to shoebury at night and it was well known to reach 100+. They would drop the pan at Leigh on sea and coast round that area as the main risk of speeding was that the pan would sway out from under the wires!

Not sure how true that is but there you go!

 

[455driver]

Don't trains all have data-recorders now that constantly log the speed?

As do most cars now, courtesy of the engine management system.

 

[notadriver]

They were I assume, using the same satellites in the same location. Couple this with the fact they may well share internal components and that most consumer GPS uses Kalman filtering, it's not really surprising.

At the end of the day it's actually an estimate.

So all three devices were wildly inaccurate then?!!

 

[WestCountry]

So all three devices were wildly inaccurate then?!!

All three devices would have been reading the same GPS signals, which themselves aren't always accurate and can be distorted by various things. So yes, it's quite possible that they'd all be inaccurate by the same amount.

You should look up the difference between 'accuracy' and 'precision'.

 

[HSTEd]

All three devices would have been reading the same GPS signals, which themselves aren't always accurate and can be distorted by various things. So yes, it's quite possible that they'd all be inaccurate by the same amount.

GPS Signal Augmentation Overlays should be able to manage considerably more accurate readings, and are becoming far more common with the drastic reduction in the price of computing hardware.

The clock corrections signals associated with StarFire (Even if you can't do the atmospheric corrections) for example can cut the error drastically.

You should look up the difference between 'accuracy' and 'precision'.

Atmospheric effects in this case are largely irrelevent, since it would tend to disrupt all readings in a general area by a very similar amount, which is irrelevent when you don't care what your position is.

If you know you moved one metre in the last second it doesn't really matter if you knew your exact initial position if all you want is the speed.

In summary, GPS speed sensors are suprisingly accurate for speeds up to a few hundred miles per hour, when you need significantly more sophisticated aviation type sensors.

 

[michael769]

Your speedo has to be in working order when new and to pass it's MOT.

Wrong. The correct function of the Speedo is not part of the MOT test

Legally if it's broken you aren't allowed to put your car on the road

This is correct.

except to take it to a pre-arranged appointment at a garage to get it fixed.

This is incorrect. You are confusing the concession allowing vehicles that have failed an MOT (or have an expired one) to be repaired and tested, with a restriction imposed by the Motor Vehicles(Construction and Use) Regulations 1989 as amended (CUR).

The latter offer no such concession. A vehicle with a failed speedo would need to be towed to a place of repair.

However, you wouldn't actually notice if it was showing 70mph when you're doing 72mph

True

until you next took the car for an MOT or service.

False. As already noted the MOT does not check the speedo, nor is a speedo check part of any manufacturers service schedule so it would not be checked unless you specifically instructed a check. Which you would have no reason to do...

According to the Highway Code the speedometer along with things including the windscreen washers, windscreen wipers and horn MUST be in good working order for your car to be roadworthy so I assumed they would be all checked as part of an MOT.

You appear to be confusing the Construction and Use Regulations, The Highway Code and the MoT test schedule.

All the MoT does is check that the items listed in the MoT Testers Handbook meet the requirements set out in the handbook. It does not check for compliance either with the Construction and Use Regulations nor the Highway Code.

There are significant differences between the law and the MoT test and it is perfectly possible for a legally roadworthy car to fail the MoT. Indeed if you have an old style MOT certificate you will find a warning to this effect printed on the rear.

 

[LE Greys]

To be honest, the A505 was the road I was originally thinking of lol. Not quite at Baldock, but not far from it!

There's a certain section near Royston with no speed cameras, and my mother knows that very well. She drives a blue Corsa.

 

[A-driver]

There's a certain section near Royston with no speed cameras, and my mother knows that very well. She drives a blue Corsa.

Come to think of it it is often a blue corsa I have trouble keeping up with around smyths farm bridge when bombing about at 90...

 

[asylumxl]

Will certainly keep an eye out for said blue Corsa.

 

[dk1]

I note the frequent mention to the damn OTMR. 150/153s could usually hold themselves around 88mph quite nicely & even faster with a 170 in the combo.

 

[TDK]

I note the frequent mention to the damn OTMR. 150/153s could usually hold themselves around 88mph quite nicely & even faster with a 170 in the combo.

But would a 150/153 be able to stop from 88mph if it had to, what would be the extra distance to stop if there was the requirement of the emergency brake to be apllied?

 

[dk1]

But would a 150/153 be able to stop from 88mph if it had to, what would be the extra distance to stop if there was the requirement of the emergency brake to be apllied?

Was only done (in my instance) on routes where line speed was higher than 75 so yes.