Speedo accuracy

[awarberg]

Do you find that the digital speedo of the Jazz Mk4 shows the true/GPS speed?

If not, what is the error, approximately?

[Kremmen]

They will rarely read the true GPS speed, my 2013 Civic is about  ~6% over.

On the M4 when I have CC set in the Smart Motorway roadworks my phone and Garmin are both showing 50mph whilst the digital speedo is showing 53mph.

If you want it spot on and have some technical knowhow you could install a 'Speedo Healer' designed for bikes but has been used on older Civics where the speedo was considerably out.

[Ozzie]

I find that on a road that has average speed cameras everyone basically does the same speed, however my 60 mph seems a bit quicker than everyone else's??
A few years ago I got pulled over by the Police for a ticking off when I over took him doing "65 in a 60", despite my sat nav showing 58 and the speedo showing 60.
The Policeman said don't trust the sat nav for speed as it doesn't allow for inclines, just distance covered on flat ground, and just wanted to hold me up for a chat, as he was peed that I overtook him 

[awarberg]

A few years ago I got pulled over by the Police for a ticking off when I over took him doing "65 in a 60", despite my sat nav showing 58 and the speedo showing 60.

On past cars I noticed that the sat nav will show less than the speedo. It makes sense since it only has GPS to go from where the speedo likely infers the speed from wheel rotation. I believe Tesla has an automatic calibration routine based on GPS which ensures that the speedo is close to true speed.

Maybe some Waze or Google Maps users have experiences to share? As these apps usually have GPS based speed readout.

[Downsizer]

A few years ago I got pulled over by the Police for a ticking off when I over took him doing "65 in a 60", despite my sat nav showing 58 and the speedo showing 60.
The Policeman said don't trust the Satnav for speed as it doesn't allow for inclines, just distance covered on flat ground, and just wanted to hold me up for a chat, as he was peed that I overtook him 

I've not thought about this before, but if I remember Pythagoras correctly, a 1:10 hill would mean that your land speed was only 0.5% higher than Satnav speed.  So if you managed 60 mph on the Satnav up a 1 in 10, your true road speed would be 60.3 mph.  As you say, your policeman was talking through his helmet!

[Jocko]

I have a GPS speedometer and it always shows the same difference between that and the car speedometer uphill and down dale. Everything I have read says the GPS speedometer is the more accurate of the two. It also matches those speed signs dotted around the countryside.

[ColinB]

Do you find that the digital speedo of the Jazz Mk4 shows the true/GPS speed?

If not, what is the error, approximately?

The car speedo will never show the true speed. That’s because the law is that a speedo must never under-read, and any over-read must be within a specified tolerance. So manufacturers will aim for an amount of over-reading that ensures zero under-read, depending on how much they are prepared to spend in the pursuit of accuracy. In the UK (from here: https://en.m.wikipedia.org/wiki/Speedometer):

“The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.[17]
The Motor Vehicles (Approval) Regulations 2001[18] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However, it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually traveling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.”

As for whether a GPS shows true road speed, you are comparing two measurement methods each with their own sources of error. The general view seems to be that GPS is probably more accurate than the car’s speedo, but whether that is truly an accurate “speed over ground” could probably form the subject of an extensive debate!

[VicW]

All the Jazz's that I have owned have indicated 2mph above the Satnav speed except at 30mph or below when they read the same 30mph. That's close enough for me. I think the tolerance on speedometers is 10%. Most speed cameras are set at speed limit plus 10% plus 2mph, ie; 30=35, 60=68. Note most but not all, police cars usually operate at the posted limit so I am told.

Vic.

[embee]

.....The car speedo will never show the true speed. That’s because the law is that a speedo must never under-read, and any over-read must be within a specified tolerance. ......

This exactly.

Satnav speeds should be as accurate as they can achieve, there is no reason (unlike with the car speedo) for it to be anything other than as good as they can get. From what I can glean, on a reasonably flat level straight road it will be pretty darned close to reality. The time factor should be spot on (they even use "relativity" to get it right), the distance resolution just depends on what the satellite and satnav technology can achieve.
I have no idea if they use elevation changes in the speed calculation, there's no reason why not. I understood satnav positioning is 3D, it is triangulating between different satellites. It shows elevation so must have a 3D log of where it is.

I've always found the differences between satnav and a specific vehicle have been very consistent, different vehicles have different calibrations obviously but each one seems consistent, so I deduce that the satnav is consistent.

[Kremmen]

Most speed cameras are set at speed limit plus 10% plus 2mph, ie; 30=35, 60=68. Note most but not all, police cars usually operate at the posted limit so I am told.

Vic.

As I understood it the 10% + 2mph is a discretionary figure. I've read about many being prosecuted via the latest digital Monitron cameras for doing true 33mph in a 30. The old wet film Gatso cameras were not as accurate so some leeway was often given.

The ones to watch out for are the latest HADECS3 cameras as found on smart motorways and the M25 that are on side shelves. They are super accurate and I've known a prosecution for doing true 71mph. The argument, when queried, was along the lines that to be doing true 71mph your speedo must have been reading ~74mph to ~77mph so you have no defence.

The other latest  ones not to mess with are the Jenoptic Specs average speed cameras. They are often dual direction in one unit and again, very accurate. They are appearing all over the place and are small yellow cameras, high up on lampposts that have what looks like a batman symbol in the bracket.

It's got to the stage where it's best to just stick to the limits.

Edit: Forgot to mention, the HADECS3 cameras are variable so if the gantry speed drops from 70mph then do likewise or they will have you, unlike the older overhead gantry cameras that only detect over 70mph.

[Jocko]

My speedo always reads higher than the GPS. 30 mph on the speedo is 27 mph on the GPS, and 60 mph on the speedo is 55 mph on the GPS. I have three GPS displays (GPS speedometer, dashcam and satnav). The satnav reads 1 mph higher than the other two, which always read the same. The speedometer and satnav both indicate altitude as well.

My Jazz speedometer of really pretty crap in that they show 20 - 40 - 60 mph, with only a mark at 30 and 50. Not exactly accurate. I refer to my GPS speedo all the time.

[Jocko]

It's got to the stage where it's best to just stick to the limits.

I watched a video explaining the technology in the camera vans and what surprised me was that they could catch you speeding from a distance of one mile, so by the time you spot the van and slow down, it is too late. The video is in the can, and you are caught.

[John Ratsey]

My Crosstar consistently indicates a speed that is 1 to 2 mph above the GPS speed. The basic speed data should be as accurate as the odometer to which a safety margin has been added. One of my previous (non Honda) vehicles appeared to be 10% high at any speed but digital technology makes it easier to use a constant safety margin.

[embee]

Bear in  mind that with digital vehicle devices there is not necessarily a direct correlation between odometer and speedometer. Old analogue things used the cable rotation directly for both through fixed mechanisms so they were fundamentally linked (although the markings could be anything), but digital processing could have any algorithm they want, one is a frequency and the other is a count. Odometer may be spot on but speedo could be anything, and not necessarily linear if they don't want.

[Kremmen]

I'm not that clever with the maths but I wonder how the mph figure would change from new tyres with 7mm of tread down to 2mm ?

Circumference x 5mm x my brain hurts !