BMW on Motorway??

[Harry Bloomfield]

NY explained :

I think that is likely them braking, because they don't even know what the speed limit is and suddenly develop a guilty conscience.

[Andy Bennet]

What do you drive? A Dinky toy?

[bert]

And also For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed?

V/10 + 6.25 mph

where V = the true speed of the vehicle in mph.

Which seems to me a very wide variation.

[tabbypurr]

maybe a Willam Sulky.

NT

[tabbypurr]

Luxury! I used to do long journeys in a thing with a wooden floor that topped out at 52mph. Eventually.

NT

[Robin]

Oh bother.

Ah, I've found where I dropped it. Here it is:

0

[ARW]

A lot down south if you count Watford as South.

The standard of driving on the M1 South of Luton is probably some of the worst motorway driving I have seen in the UK.

[alan_m]

What about different minor variants of the same basic car? It wouldn't be unusual for them to be fitted with either,say, 16 or 17 inch wheels. Do the manufactures adjust the calibration for different factory fitted wheels or do they rely of range they allowed on the speedo readings?

What about the reading within spec with a space-saver wheel on one corner

[Steve Walker]

One guy I worked with drove from Manchester to deepest Cornwall and back in one of those small, 4-wheel, rear-steer cranes with a top speed of

12mph !!!

SteveW

[Dave Plowman (News)]

Really?

So you start out with a speedo dead accurate with new tyres. Then the maximum error would be 2%. Not 10% as allowed. Things have moved on since that reg was written.

[FMurtz]

Same external diameter.

[FMurtz]

They start out never true speed reading,they always read less, except for police speedos in police vehicles.

[FMurtz]

You lot are lucky,here in Australia we have just made a law that any flashing police or emergency vehicles at the side of the road means that you have to slow to 40 kmph which means that you have to reduce speed from 110 kmph to 40 kmph suddenly on the highway, it has already caused accidents.

[The Natural Philosopher]

I mentioned ths top a plumber. His tale was interesting.

"Most people dont know what the speed limit is, so around town they drive at 20mph to be safe, and out of town they drive at 50mph".

[Andy Bennet]

All depends on the rolling radius of the tyre , which is not only due to the tread depth but mainly the tyre inflation pressure. A fully inflated tyre should give the most accurate reading. Fortunatly the flatter the tyre the more the speedo over reads.

I think the fuzz now use a laser velometer fitted under the vehicle which measures the true road speed relative to the vehicle.

[Andy Bennet]

Only trust the satnav speed when on a level road. The speed is calculated by rate of change of position on a flat surface. If you are going up or down a hill it will under read.

The derivation of altitude via GPS is not very accurate anyway and is ignored in domestic satnav devices.

[The Natural Philosopher]

A one in ten hill is so steep that most people would slow right down on it.

A one in ten hill would underead by 10%. In essence practically negligible on any normal slope of 1 in 100 or so.

[Andy Bennet]

M25 going downhill 10% near Reigate golf course:-

[Harry Bloomfield]

Andy Bennet formulated on Thursday :

Yes, it does make a difference, but for all practical purposes can be ignored. The major difference to be taken into account, is acceleration and deceleration - they provide accurate readings only at steady speeds.

Correct, mine shows the altitude, but rarely moves from 114 feet ASL irrespective of where it goes.

[Andy Burns]

Mine shows true speed minus 1 mph, compared to GPS speed and to those annoying signs that flash to thank you for being considerate.