GPS or Speedometer Readings?

[Brian WH]

Quote from another thread by GrahamC230K.

Our speedo's are more reliable than most.
Measuring against the GPS on my Road Angel (accurate while at a steady speed), I have found that my MB speedo only over reads by 2-3mph from 30mph to flat out.

I have found that mine is similar, about 2mph less than Speedometer when set against my Garmin GPS.

So question is "Which is Accurate", the GPS or Speedometer? The GPS is only good for 20ft so how good is the speed reading?

Discuss with proof positive?

 

[miro]

Quote from another thread by GrahamC230K.

Our speedo's are more reliable than most.
Measuring against the GPS on my Road Angel (accurate while at a steady speed), I have found that my MB speedo only over reads by 2-3mph from 30mph to flat out.

I have found that mine is similar, about 2mph less than Speedometer when set against my Garmin GPS.

So question is "Which is Accurate", the GPS or Speedometer? The GPS is only good for 20ft so how good is the speed reading?

Discuss with proof positive?

The GPS samples the total displacement of the car over an extended period and is therefore VERY accurate. The error factor is negated by the many samples and the average distance between points compared to the distances possible in the map.

Even if the GPS is out by 20ft you know perfectly well that a left turn at a road intersection isnt out by 20ft because no turn can be that wrong. So in this way the GPS compares known map data to GPS data and attains an almost perfect reading of distance and time = velocity.

The GPS with speedpulse can also improve this reading.

 

[jimmy]

The Road Angel and Snooper GPS are both claimed to be something like 99% accurate, in all the cars I have tried the Road Angel and Snooper the speedo has always read slightly more.

Both the Snooper and the Road Angel read exactly the same as the Travelpilot GPS, I would say that the GPS is more accurate than the speedo.

 

[Anders]

I have found that my speedometer is scarily precise, i.e. within 1-2 km/h when comparing with the GPS reading.

This means that the indicated 270 km/h I saw in Germany is actually true! :bannana:

 

[PJH]

By Law a vehicle's speedometer must not under-read, so they err on the positive.
As GPS appears to need a certain amount of movement before it gives a reading it must therefore be giving an average speed not an instant reading.
If you know of a stretch of dual carriageway with marker posts in the central reservation you could set your cruise at say 75mph (120kph) and time yourself over 1 or 2 km.
The said marker posts are 100metres apart.

I only chose 120kph 'cos it's easy to work out, 2km per min.

 

[Aswall]

I have 2 GPS devices in the car (Tomtom and Roadpilot) and they both read exactly the same all the time showing the speedo to be over by 2mph at 30mph up to 6mph over at a steady 100mph (abroad naturally ) so about 6% over which is within the limits allowed I think

 

[sym]

With 17" wheels on the 202, Speedo now reads almost exactly the same as the Snooper GPS. Previously, with 16" wheels, the difference was about 5-10% - speedo reading more. Must be something to do with the slightly different rolling radius - even though it should be roughly the same, there must be a degree of difference, within normal tolerance, that affects the speedo reading marginally. Will see what happens when my new(ish) tyres wear down a bit !

S.

 

[Will]

With 17" wheels on the 202, Speedo now reads almost exactly the same as the Snooper GPS. Previously, with 16" wheels, the difference was about 5-10% - speedo reading more. Must be something to do with the slightly different rolling radius - even though it should be roughly the same, there must be a degree of difference, within normal tolerance, that affects the speedo reading marginally. Will see what happens when my new(ish) tyres wear down a bit !

S.

I am glad that sym mentioned that. On that note, speedometers on cars would be less accurate as they are relying on revolutions of the wheel(s) to calculate a speed measurement. Surely, as tyres wear down from 8mm of tread, down to the 1.6mm or so minimum, this would affect the reading to some extent? (ie, the more they wear, the higher the speedo would read?) Also, uneven road surfaces may have a slight affect (we are talking about accuracy, right!)

I suppose my final point would be that because of this "open loop" system of "measurement", the speedo in no way "measures" your speed, rather more it "estimates" your speed.

In contrast, the GPS has only one real variable; your car! If it knows your relation to fixed bearings, and the time it has taken you to reach one point from another, it could give you an accurate speed calculation. Yes, this may not be totally instantanius, but should be more accurate in general than a system that can be fooled and guesses your speed, as well as having to err on the high side to cover the manufacturers back!

 

[Brian WH]

Well all the answers are feasable, any more comments before I start to believe my GPS????

The tyre wear and other speedo factors are true but surely marginal?

You would have thought that MB would have got it all sussed by now wouldn't you.

 

[GrahamC230K]

Well all the answers are feasable, any more comments before I start to believe my GPS????

The tyre wear and other speedo factors are true but surely marginal?

You would have thought that MB would have got it all sussrd by now wouldn't you.

Unless your GPS is a poor example, then you should be believing it.

Tyre wear is a marginal factor, but when we are disputing 1pmh of accuracy, it's a worthy point.

I think MB have got it sussed. From reading reviews of other cars (I think Autoexpress or someone did an article I read) I know some vehicles are way out. Within normal UK speeds there are not too bad, but the innacuracy seemed to be a percentage of the real speed, so by the time the car was flat out in the test conditions, the speedo reading was wildly out.

As I say, my speedo seems to only read 3mph over, from the lowerst speed I have tried to maintain - 30mph up to cruising all out on the autobahn.


Maybe like Sym, me having the 17" wheels has inadvertantly made my speedo slightly more accurate than it would have been with 16" wheels.

 

[Brian WH]

Unless your GPS is a poor example, then you should be believing it.

Tyre wear is a marginal factor, but when we are disputing 1pmh of accuracy, it's a worthy point.

I think MB have got it sussed. From reading reviews of other cars (I think Autoexpress or someone did an article I read) I know some vehicles are way out. Within normal UK speeds there are not too bad, but the innacuracy seemed to be a percentage of the real speed, so by the time the car was flat out in the test conditions, the speedo reading was wildly out.

As I say, my speedo seems to only read 3mph over, from the lowerst speed I have tried to maintain - 30mph up to cruising all out on the autobahn.


Maybe like Sym, me having the 17" wheels has inadvertantly made my speedo slightly more accurate than it would have been with 16" wheels.

I wasn't disbelieving it mate, just debating the issues surrounding the technology. Mine is like yours, the same difference throughout the range.

 

[PJH]

Another 2p worth

As I say, my speedo seems to only read 3mph over, from the lowerst speed I have tried to maintain - 30mph up to cruising all out on the autobahn.


Maybe like Sym, me having the 17" wheels has inadvertantly made my speedo slightly more accurate than it would have been with 16" wheels.

Although the rolling circumference may be almost identical at a walking pace, the difference in sidewall depth may give a greater difference at top speed.
i.e. one tyre may 'grow' more than the other due to centrifugal forces.

I would think a 5% over-reading of the mechanical speedo is within manufacturing limits.

 

[JumJum]

Most cars used to over read by quite a bit, I remember a friend getting stopped on the A74M years ago doing what he thought was 120mph in a 2 litre Sierra, he was very upset to discover that the police had caught him doing "only" 99mph

This was not uncommon to see really quite strange readings used to drive a 1000cc Mini that downhill would wind round right past all the speedo numbers to Full on the fuel gauge ! This should have been equivalent to about 100mph, but still Grannies in Morris 1100s would overtake me.


My car seems to be quite accurate, the limiter is set at 250 kph, 156-7 mph and when the car hits it it just stays reading a steady 160mph.

 

[sym]

My car seems to be quite accurate, the limiter is set at 250 kph, 156-7 mph and when the car hits it it just stays reading a steady 160mph.

....can almost see the blur of your fuel gauge needle falling on that photo !!

 

[mapleleaf]

I've got the B2 and noticed that the speedo 's on th ML & SL always over read comapared to the GPS by 8-10% - the Aston is worse still. At higher speeds the margin increases. eg 40mph on speedo shows as around 36-37 on the B2

the acid test is driving past past speed cameras at an indicated speedo reading of 5 mph above the limit and not getting flashed!

In a way its a good thing as if you rely on your speedo to comply with road limits you will never be a marginal case or worse slightly over.! particularly if plods following.

could be fun though relying on the GPS reading with a following plod & driving exactly on the posted limit.

 

[PJH]

the acid test is driving past past speed cameras at an indicated speedo reading of 5 mph above the limit and not getting flashed!

Yes, off you go then Steve, let us know the results.

PS Gatsos don't work above 170mph. :bannana: :bannana:

 

[jimmy]

could be fun though relying on the GPS reading with a following plod & driving exactly on the posted limit.

I did exactly this a few weeks ago, got off the M5 at Cirencester and had a traffic car following me all the way across the Cotswolds through Stow on the Wold, Chipping Norton etc. Most of that road is now 50mph which is a great shame as it is a nice stretch of country road. I stuck dead on 50mph according to my Snooper which is about 54mph on the speedo, a few times downhill I let the speed creep up to nearly 60mph or so before very gently braking. A couple of times it felt like the police driver was trying to push me a bit further, almost egging me on to be caught.

 

[Aswall]

Yes, off you go then Steve, let us know the results.

PS Gatsos don't work above 170mph. :bannana: :bannana:

All the Gatsos and Truvelos round my neck of the woods are set at 10mph over anyway so not a prob

 

[Brian WH]

Data from a GPG Information service.

GPS Speed:
How accurate is it? GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.

HOW ACCURATE IS THE SPEED READING?

From the NAVSTAR GPS User Equipment Introduction document Section 3.7:

GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.

Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.

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The only thing that I will add, having tried mine recently, is that this is fine in a straight line. If you drive at constant speed down twisting country lanes, the GPS speed is up and down all the time.

 

[300eDAVE]

Most cars used to over read by quite a bit, I remember a friend getting stopped on the A74M years ago doing what he thought was 120mph in a 2 litre Sierra, he was very upset to discover that the police had caught him doing "only" 99mph

QUOTE]

More likely they were just being decent people and said it was 99mph so that he wouldnt get the obligatory ban for being 30mph over the limit.

And maybe its less paperwork for a traffic cop to nick him for 99mph rather than 110 ??