Speedo Accuracy

[ButtonIt]

Has anyone worked out the accuracy of the MINI speedo? If you use a GPS type device will that tell you the exact speed you're travelling at?

So for example....

80mph on the mini speedo (digi readout) running 17" wheels would read out as what on the GPS device? and how accurate is this to your actual speed?

[ALD0]

The GPS will show about 78 when doing 80-The car speedo always seems to be a few mph faster.
Even still the GPS is more accurate aswell.

[Wobert]

All measuring devices will have a tolerance on their accuracy.

Car speedometers are only allowed to have a +ve tolerance, i.e. they are not allowed to under-read, and can only over-read by 10% max.

GPS devices will also have a tolerance, due mainly to the accuracy of their positioning.

The GPS, we civilians are allowed to use has a tolerance on position of +/-5m.

Once this tolerance is "triangulated" i.e. how the receiver works out your position and rate of position change, then obviously this then affects the accuracy of the speed read-out.

[Goonery]

Roadangel tells me 77 - 78 when speedo says 80, it seems progressively more accurate, so 30 on the GPS will be about 26 on the speedo.

I guess there is a reason behind the question ?, but I'm hoping its not what I think it is

[ButtonIt]

M4 by langley junction there was a camera unit at the side of the road my cruise control was set to 80 (all lanes were flowing at the same speed and failry few cars about) saw it and gave a burst on the anchors to drop to about 65..... just not sure of the range on them buggers or how accurate the speedo is, or the tolerance of the camera

[Wobert]

Quote: Originally Posted by ButtonIt

M4 by langley junction there was a camera unit at the side of the road my cruise control was set to 80 (all lanes were flowing at the same speed and failry few cars about) saw it and gave a burst on the anchors to drop to about 65..... just not sure of the range on them buggers or how accurate the speedo is, or the tolerance of the camera

The police cannot prosecute you, provided you are within the 10% margin of error on your speedo.

So in a 70 mph limit, they cannot "do" you for 77mph.

Since most GPS speeds are more accurate than the speedo reading (at a constant speed, i.e. not whilst accelerating).....I'd guess you'd be OK.

[Neil the Hat]

Quote: Originally Posted by ButtonIt

M4 by langley junction there was a camera unit at the side of the road my cruise control was set to 80 (all lanes were flowing at the same speed and failry few cars about) saw it and gave a burst on the anchors to drop to about 65..... just not sure of the range on them buggers or how accurate the speedo is, or the tolerance of the camera

Range can be 1000 metres plus on a speed camera.

I also don't think the excuse that my car was reading a legal speed but you did me for 77 plus, works with the majority of Officers. It is the driver's responsibility to have their speedo calibrated to the correct reading.

Think the 10% rule doesn't really wash much these days, especially if the car has been manufactured in the last few years.

It is the discretion of the Officer as to whether he 'does' you for 71 or 77, or what level he sets his camera at, which could be 75, 80 or whatever.

Lastly, it maybe the local policy to only prosecute the top percent of speeding drivers, if they have caught a few hundred they may only prosecute 10%.

It's a Lottery...

[Wobert]

Quote: Originally Posted by Neil the Hat

Range can be 1000 metres plus on a speed camera.

I also don't think the excuse that my car was reading a legal speed but you did me for 77 plus, works with the majority of Officers. It is the driver's responsibility to have their speedo calibrated to the correct reading.

Think the 10% rule doesn't really wash much these days, especially if the car has been manufactured in the last few years.

It is the discretion of the Officer as to whether he 'does' you for 71 or 77, or what level he sets his camera at, which could be 75, 80 or whatever.

Lastly, it maybe the local policy to only prosecute the top percent of speeding drivers, if they have caught a few hundred they may only prosecute 10%.

It's a Lottery...

Er not quite......

Current vehicle homologation rules state that the speedo is not to under-read. It can only over-read to a maximum of 10%.

On that basis the police cannot enforce a speed prosecution within that 10% margin of error.

So they cannot "do" you for 71 mph in a 70 mph limit, as this is within the tolerance set by vehicle legislation. The best they can "do" you for is 78 mph.

As far as I'm aware, the only vehicles which have "calibrated" speedos, are emergency vehicles, and HGVs, where the speedo signal is used to govern the maximum speed to 90 kph (56 mph).

[m19dut]

Don't hold me responsible if you get a ticket but... I was led to believe by a source that they give you 10% plus 2mph. I think you are more likely to be done if you are speeding in a residential area/school etc but on the motorway I think there is more le-way.

[ButtonIt]

I didn't start this thread with a "will I/ won't I get a ticket" argument in mind. I'm resigned to the fact that if I get a ticket I'll have to pay the consequences, there's no dispute there.

I'm just interested in the tolerances of the devices used and how they differ from the information given to the driver.

Another thing I'm quite interested in finding out is if wheel sizes (MINI standard wheels) have much of an effect.

80mph on speedo with 15's
80mph on speedo with 16's
80mph on speedo with 17's

and so on

[KPKalle]

Quote: Originally Posted by ButtonIt

I didn't start this thread with a "will I/ won't I get a ticket" argument in mind. I'm resigned to the fact that if I get a ticket I'll have to pay the consequences, there's no dispute there.

I'm just interested in the tolerances of the devices used and how they differ from the information given to the driver.

Another thing I'm quite interested in finding out is if wheel sizes (MINI standard wheels) have much of an effect.

80mph on speedo with 15's
80mph on speedo with 16's
80mph on speedo with 17's

and so on

I'd be interested in an accurate answer to that. We have to change to winter wheels soon - from 16s, 17s or whatever to what's normally 15s for winter. No-one, I'm sure, recalibrates anything to make allowance for speed variance.

Anyway I'm pretty convinced I get better fuel economy in winter. And not just because of slightly more conservative driving

[Wobert]

Quote: Originally Posted by ButtonIt

I didn't start this thread with a "will I/ won't I get a ticket" argument in mind. I'm resigned to the fact that if I get a ticket I'll have to pay the consequences, there's no dispute there.

I'm just interested in the tolerances of the devices used and how they differ from the information given to the driver.

Another thing I'm quite interested in finding out is if wheel sizes (MINI standard wheels) have much of an effect.

80mph on speedo with 15's
80mph on speedo with 16's
80mph on speedo with 17's

and so on

I thought I'd tried to answer that with my first post.......

[ButtonIt]

you did in a way wobert I'm interested in actual figures though, we might as well get some physical readings. There's enough MINI2'ers out there with the equipment to do it

[Wobert]

Quote: Originally Posted by ButtonIt

Another thing I'm quite interested in finding out is if wheel sizes (MINI standard wheels) have much of an effect.

80mph on speedo with 15's
80mph on speedo with 16's
80mph on speedo with 17's

and so on

Now are those speeds with "tyres new" or "tyres worn" as both conditions will affect the reading at the speedo.

In my last job, designing gearboxes for JCBs, we used to factor in all the permutations for the various tyres available to suit each machine type. Allowing for some sizes, would mean that a JCB 3/4CX (digger to you lot!) could theoretically exceed it's persmissible speed allowed by legislation.

[Wobert]

Quote: Originally Posted by ButtonIt

you did in a way wobert I'm interested in actual figures though, we might as well get some physical readings. There's enough MINI2'ers out there with the equipment to do it

But using GPS, speed gun or any other device, will still have a tolerance on it.

The only true method is to time the car / vehicle over a calibrated distance, and work the speed out from that.