Speeding and displayed speeds

[@MARK]

There are a couple of speeding related posts on here this week however I have discovered something myself over the last few days which has surprised me.

On previous cars the speedo has always been inaccurate. This is shown up when I have fired up a Tomtom where the speed displayed is accurate to a very high degree. Typically this has shown the speedo on the car to be close to 10% adrift (always fast) so that where the speedo is showing 77mph, you are actually doing closer to 70mph.

As such, when driving on journeys where I have been using the Tomtom, I have always used the sat nav speedo (especially when setting cruise control) to eek that little bit more speed out whilst still staying legal.

This appears to be a common thing and on most forums, this is accepted to be the norm.

So my C350 is new and I am still getting the feel for it but I have noticed that when cruising at a displayed 70mph, I do seem to be going slightly faster than other traffic. This is noticable also in roadworks where the limit is 50mph and I find myself going slightly faster than other traffic much the same as in the past where I have used the GPS to set the speed.

So last week, I managed to fly through a mobile speed trap hiding on a bridge as I drove down the A40 somewhere south of Ross on Wye. My speedo was a tad below 80mph so whilst I did lift off, I was not to worried.

But then I started thinking. How accurate is the displayed speed on my new car?

So armed with a Tomtom on the dash and a GPS based speedo app on my phone I set out to test.

Even at very high speeds, the speedometer was never more than 2mph adrift. So at a displayed 80mph the GPS reported 78.5mph!!!

So is this common across MB as a whole. Do they differ from all other car manufacturers (including BMW, Ford, Volvo and every other car I have ever owned)?

And are you guys all driving around thinking you have a 10% tolerance?

PS

Still within the 14 days so no idea if I got done or not the other day (such is life)

 

[Deleted member 6183]

I always found that that my MBs over read by about 3mph, current BMW by 2mph.

Hope that you weren't clocked in the 50mph limit at Pencraig, a few miles South of Ross.

 

[MicB]

About 2.5% over read on my SL, ie. when the car claims that it is doing 100mph it is actually only doing 97mph-98mph.

Mic

 

[corned]

As with all the other speeding and camera threads, you will find that assumption is the sport of fools.

Not that I'm calling you a fool, but it is always best to find out for certain, or leave safe margins rather than pushing it.

 

[Stratman]

I tested my speedo and COMAND with my Garmin Etrex and at an indicated 70mph (the speedo and COMAND agreed with each other) the Etrex said 68.7mph.

I'll live with that.

 

[jefrs]

The speed from a GPS Satnav is not as accurate as you think it is, they do have a better resolution, but that is not the same as accuracy.

The Satnav has to do some fancy averaging and trigonometry, vector your direction as a straight line and calculate your distance travelled in time taken. Whereas your speedo is connected to the rotation of your wheels, and that depends on the circumference of your tyres, which varies, but you do get an instant velocity reading. The Satnav always has to lag behind real time, and gives an average speed. The sensors for speedos have much improved over slipping clutch systems of olde.

You do not have military GPS, so your location can be out by many metres and also depends how often position is updated. The system also has to know exactly where the satellites are, and they do vary, quite a lot sometimes.

 

[Stratman]

Agreed, but the Etrex isn't a satnav, it's a pure GPS receiver. It can spend all its time and computing power on the important stuff rather than on drawing fancy graphics. Its accuracy specification for steady state velocity is 0.1 knot, which is 0.115 mph.

 

[st4]

Depends on each car, speedo and wheels used. The shed over reads by 2mph at 80 the old car 3mph

 

[@MARK]

As with all the other speeding and camera threads, you will find that assumption is the sport of fools.

Not that I'm calling you a fool, but it is always best to find out for certain, or leave safe margins rather than pushing it.

I wasn't making any assumptions at all hence checking to find out for sure.

Previous cars have always read over when checked that's all. I am not for one second sitting here claiming that it was the fault of the speedo reading fast.

In this case, I was pushing on. Not a lot but if the ticket lands I can't (and won't argue about it). Also I know I was in a 70 limit as I passed a couple of very temporary NSL signs and wondered why they were there, then I looked up onto the bridge and got my answer.

 

[prprandall51]

The speed from a GPS Satnav is not as accurate as you think it is, they do have a better resolution, but that is not the same as accuracy.

The Satnav has to do some fancy averaging and trigonometry, vector your direction as a straight line and calculate your distance travelled in time taken. Whereas your speedo is connected to the rotation of your wheels, and that depends on the circumference of your tyres, which varies, but you do get an instant velocity reading. The Satnav always has to lag behind real time, and gives an average speed. The sensors for speedos have much improved over slipping clutch systems of olde.

You do not have military GPS, so your location can be out by many metres and also depends how often position is updated. The system also has to know exactly where the satellites are, and they do vary, quite a lot sometimes.

The differential between military and civilian sat navs no longer exists (it was eliminated in 2000). Any domestic GPS receiver is now as accurate as a military unit and both go down to only a few metres accuracy. The only way to improve on a GPS accuracy is to employ differential GPS data, which is available to civilians, too.

When measuring speed with GPS, it is the position variance that indicates speed, not the absolute position so even those few metres of inaccuracy become irrelevant as the inaccuracy is consistently applied (if you see what I mean); as a result,a GPS unit gives a very accurate speed reading on a flat road, although steep hills will generate errors because a GPS cannot detect height variances accurately.

For the purposes of calibrating a speedometer, a GPS used on a relatively flat, straight road will provide a very accurate reading.

 

[Dieselman]

Also I know I was in a 70 limit as I passed a couple of very temporary NSL signs and wondered why they were there,

They are the joke ones the local kids have been putting up over Christmas.

Little Buggers...

 

[Benzster]

Must check when I bought my GPS gadget!!

 

[prprandall51]

Must check when I bought my GPS gadget!!

If you are thinking pre- or post- 2000, it doesn't make any difference; the change was to the satellite signal, not the receiver technology, which remained unchanged.

From my limited personal experience (based on my own GPS receivers), receivers are no more accurate nowadays than 10-12 years ago, but their speed to acquire signals, ability to retain signals and ability to lock onto weak satellite signals has vastly improved in the last few years.

 

[Mercy1]

My car speedo reads 3 mph faster than my Garmin - and I believe the GPS is most accurate. I always set my cruise control to 78 mph on the Garmin, or 55 mph in 50 mph stretches - and keep my fingers crossed! . I do usually ease off a fraction when Garmin warns of possible speed camera locations...(no tickets so far, touch wood!)

 

[This is Pete]

I'm a somewhat simple guy and I tend to work to the indicated speed on the speedo. I know it's going to be lower than the actual speed to some extent, so that's my safety buffer.

Placing my trust in an after market GPS device (inherently and purposely inaccurate) means I'll have no recourse if flashed or ticketed. I can't really see TomTom, Garmin, or any other GPS manufacturer sending a representative to stand in court and defend the accuracy of the speed indicated by their device.

 

[A210AMG]

I think mine is pretty accurate to iphone / GPS.

I would have never assumed 10%... I would like to think if I'm doing 70mph I'm doing 70mph...

Good luck with the post

 

[WillDeBeest]

Reviving an old thread because I'm new to MB and built-in speedometer error is something I've noticed (and measured) in my previous cars.

So in both the Volvo S60 we still have and the Toyota Verso the S211 replaced, a true 70 mph reads as about 75 on the dial. I don't have a GPS or satnav - although I have used a little free iPhone app called 'Speed', which used to work well and no longer does. Much better was timing the car at constant speeds along large numbers of 100m posts on the M40, which I quite often used to drive late in the evening when traffic was light, and this is where my data comes from.

I no longer do those trips on the M40, so it's taken me a while to calibrate the E220's speedo, but I got the perfect chance in France last month. On a 130 km/h stretch of motorway, Mrs Beest was driving with the CC set at an indicated 114 km/h or 70.8 mph. Traffic was so light that she kept this up long enough for me to time 10 km at a constant speed, which we covered in 321 seconds giving a true speed of 112.1 km/h, or 69.7 mph. So an over-read of 1 mph, rather than the 5 mph I'm used to.

I've not had such a good timing opportunity since, but I did several single kilometres at similar speeds and they all came in just as close. Of course, there may be variation between the instruments fitted to different examples of the same car (mine dates from 2008-09) but this does seem to be much the most accurate I've had, at least at motorway speed, and consequently I am treating it with due respect!

 

[finisterre]

Mine was 2.8% slow. (sat nav and milepost timed)

I have changed my wheel diameter so the speedo is spot on. +/- 0.3% depending on tread depth.

It bugged me it being wrong.