How accurate is your Speedo?

[mattsurf]

Just got a new Satnav system, one of the features shows your speed. On a trip from Weymouth back to Sutton Coldfield yesterday, I was interested to see how accurate the Speedometer on the car was. Set the cruise control at 70, Satnav showed 69.1 mph, set the cruise control at 80, satnav shows 78.3mph. My speedometer would appear to be 98% accurate, which I think is pretty good. Tested my wife's Honda, at 70mph, the satnav indicated 66mph (the tyre tread is a little low, which probably explains the difference).

I have noticed that I seem to be quicker through the speed camera's on the M6 than most other traffic, even though I stick to the limit (within 2-3 mph of the limit). Could it be that Mercedes Speedometers are more accurate?

 

[television]

I think that MB speedometers are more accurate than most, many small cars are way out indicating a far higher speed

 

[BlackC55]

MB speedo's are very accurate. Just ask the old bill.

 

[philharve]

Hi mattsurf

I seem to recall reading that speedometers are designed to read on the high side, somewhere between 3 - 5 percent, so that at an 'indicated' 70mph, the actual road speed is in the high sixties. They are not, by Law, allowed to function the other way round, i.e. be low reading, for obvious reasons. The police also apply a rule when assessing speeding convictions which, if I recall correctly, is 10 percent plus 2mph. Thus a speed limit of 70mph translates to 79mph which, if measured by laser monitoring equipment (say), could result in a conviction. A high reading speedo' should ensure a conviction is avoided provided the indicated speed doesn't exceed local restrictions. Have you noticed that the average speed of cars on British motorways is somewhere between an indicated 70 - 80mph? Technically, many of these drivers are pushing this upper limit and are travelling above 70mph. I have often seen vehicles travelling way beyond this limit and a few get caught.

The GPS has inaccuracies too. The in-car navigational systems rely upon detecting the signals from a number of earth-orbiting (polar) satellites and then performing a series of quick triangulation calculations to determine the vehicles approximate position on the surface of the Earth. The more satellite signals that are detected the more calculations that are performed and the better the prediction of the vehicles true position. At best, this location can be determined within a few metres. Since the vehicle is moving, its position is not static (obviously) and it's the rate of change of position that determines the vehicle's velocity. If the in-car GPS takes 2 position readings, 10 seconds apart, and neither is an accurate location reading, you can appreciate that the GPS-indicated velocity will be an estimate too.

In bad weather, heavy rain (say), the water molecules in the clouds and in the rain drops block the satellite signals and only the strongest ones are detected by the in-car GPS. When the number of detected satellites fall, the positional accuracy degrades and consequently the GPS-indicated velocity degrades too. It loses its accuracy. At least 3 signals are required for GPS to work at all. Four signals is often the minimum required by most GPS systems to acquire satellite synchronisation.

I good weather and when receiving many strong signals, the in-car GPS-predicted velocity is probably more accurate that the vehicle's speedo' reading. But it doesn't stay like that because the weather in the British Isles is unpredictable.

Are Mercedes speedos more accurate than other makes? Probably not. But speedo' readings are only estimates anyway. If you stay within the indicated speed and that speed does not exceed any locally imposed restrictions, your driving license should stay free of endorsements.

If you are using your vehicle's speed control system during your testing, remember that this indicated speed will not entirely accurate either. What you are attempting to do is to compare the accuracies/innaccuracies of different speed-recording equipment. Each has its advantages and disadvantages.

If you then apply Einstein's theory of relativity then no recording/measurement we make is truly accurate, only relative.

REGARDS

Phil

 

[television]

Then use the white markers on the A roads and motorways with a stop watch and do allow for your thinking time, mind you this should cancel out if the same person pushes the start and the stop

 

[Devonian]

Presumably on vehicles with smaller wheels then the tyre tread wear would have a larger effect on the speedo variation as the percentage of wheel size change would be greater
Also bear in mind that sat nav speed readings work by calculating the position of the vehicle as seen from space relative to time but takes no account of road gradients. Therefore on perfectly flat ground they would be accurate but on gradients they will in fact show a lower speed than your actually travelling at.

 

[television]

Presumably on vehicles with smaller wheels then the tyre tread wear would have a larger effect on the speedo variation as the percentage of wheel size change would be greater
Also bear in mind that sat nav speed readings work by calculating the position of the vehicle as seen from space relative to time but takes no account of road gradients. Therefore on perfectly flat ground they would be accurate but on gradients they will in fact show a lower speed than your actually travelling at.

Good point there

 

[Myros]

Malcolm's post marker method

16 posts ( don't count the first one, just set your clock off as you pass it, and stop it as you pass post 16) = 1600 metres, = 1mile
10 posts = 1000 metres =1 km. on very curved sections of motorway, this does induce an error if you are on the inside or outside of the curve. If you can manage to keep your desired speed in lane 1, then the error is minimised.
Parellax errors increas as you get further from the posts, ie into lane 3 or 4.

Best to have your observer/navigator/kids or missus do it for you. I have done it on all my cars and found the speedos to be very close to the expected mark. However, all the cars I have owned have had VDO or Motometer instruments and that says a lot.

You can also do quick cross checks by working out mph/1ooo revs and then checking against your rev counter.

 

[rf065]

In my experience, Mercedes speedos are definately more accurate than some other makes.

I have used the same TomTom unit in various cars, one typical example being a Nissan X Trail. Measuring the speedo at 60, 70, 80 & 90mph, would always give a reading on the SatNav, 5mph less. This has been compared over a prolonged time scale, so depth of tyre tread etc has made no difference that was noticable, nor has travelling up or downhill. In the 5 years I owned the car, it was always 5mph less.

I checked the speedo in the Mercedes, and the same SatNav shows it is only 2mph less until you hit 90 and over when it shows 3mph less.

Russ

 

[television]

In my experience, Mercedes speedos are definately more accurate than some other makes.

I have used the same TomTom unit in various cars, one typical example being a Nissan X Trail. Measuring the speedo at 60, 70, 80 & 90mph, would always give a reading on the SatNav, 5mph less. This has been compared over a prolonged time scale, so depth of tyre tread etc has made no difference that was noticable, nor has travelling up or downhill. In the 5 years I owned the car, it was always 5mph less.

I checked the speedo in the Mercedes, and the same SatNav shows it is only 2mph less until you hit 90 and over when it shows 3mph less.

Russ

My 129 was the same as yours, I have not tried the 230

 

[Ade W124]

mine reads about 10% low according to my GPS...

indicated 90 is about 80, 45 is 40 etc.. Its on standard wheels with decent tread but the needle is a bit wobbly below 40 which suggests something is worn.

Does this also mean that the odometer is reading low?

Ade

 

[television]

mine reads about 10% low according to my GPS...

indicated 90 is about 80, 45 is 40 etc.. Its on standard wheels with decent tread but the needle is a bit wobbly below 40 which suggests something is worn.

Does this also mean that the odometer is reading low?

Ade

Hard one to say, the needle shows that the speedo cable needs some lubricant, the mileometer is hard geared

 

[David Pemberton]

My RoadAngel (which uses GPS) shows 77 mph when my speedo shows 79 mph. And that's where I set my cruise control per the comments above about the way the police tend to view things (if you aren't driving like a fool too!).

My Polo is very near the 10% over-reading tolerance limit. Not tried the SL to see what Mercedes speedos were like 20 years ago.

I drive through a Specs (average speed) controlled zone on the M1 every day. I set the cruise on 50mph per the RoadAngel and am usually passing everyone at a very gentle rate - it's a combination of their over-reading speedos and most driver's inability to match a cruise-control for keeping a steady speed.

David

 

[rf065]

My Polo is very near the 10% over-reading tolerance limit. Not tried the SL to see what Mercedes speedos were like 20 years ago.

See below, but even nearing the 10% limit you are still well within accepted accuracy limits.


"Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed."

Russ

 

[mattsurf]

The reason that I was thinking of this in the first place was that a mate was following me in his Mondeo on the M6 Junction 4 to Junction 9 (Loads of cameras there). I had the cruise on 73/74 mph - my friend reconded that I was well over 80 according to his speedo. Now that I have a GPS (given that this is not 100% accurate) I know that I was probably doing in the region of 72/73mpg, so not putting my licence at risk.

Anyway, it is interesting that Mercedes have more accurate speedometers than other cars. Why do other makes have read so high?

 

[verytalldave]

Several roads in my area have the new speed warning indicators fixed to lamposts. They show the vehicles speed on a large digital readout. These devices are perceived by many to be more effective at reducing speeds than speed cameras. I agree, however thats not the point. I have 2 cars, a 52 reg Clio and a 54 reg C200K. As you approach the indicator located in Leesons Hill in my Clio, if you drive to get the indicator displaying 30, then my Clio's speedo is showing about 34/35 mph. Going through at the same display at 30 in my C200K, it shows about 31/32 to the best of my judgement.
Therefore, I tend to agree with the suggestion that Merc speedos ARE indeed more accurate. :lol: :lol: :lol:

 

[television]

I think many little cars speedometers read fast so that the owners can tell people how fast their car car goes, like my friends Nissan Micra 12 years old and goes 135 MPH

 

[verytalldave]

I think many little cars speedometers read fast so that the owners can tell people how fast their car car goes, like my friends Nissan Micra 12 years old and goes 135 MPH

LOL ! ! ! ! ! ...........It probably would do if it was driven by my next-door neighbours son......

 

[television]

LOL ! ! ! ! ! ...........It probably would do if it was driven by my next-door neighbours son......

I like it

 

[Schtum]

Mine reads 5mph high at 70mph according to Mr TomTom. Whom am I to argue?