Speedometer reliability / accuracy

[Tailhappy]

Back in the 70's and 80's magazines like Autocar used to test car speedometer accuracy across the range. Years ago when I had my Skoda Rapid 136 I just subtracted 10% off everything (got that from the Autocar review) to get the correct reading. 

 

But now this information is missing from Autocar and not available online that I can see either. I use a lot of hire cars and when I'm on a clear road I do a quick check against sat nav so I understand if there is any discrepancy. There is a lot of variation with the digital displays not making much improvement either. 

 

Is there any magazines or website reviews that still list this info? Would be good if there was.

 

Have you ever tested your car against sat nav value? What was the offset? While sat nav isn't 100% reliable it does match the roadside speed displays which are pretty accurate. 

 

 

[MikeTheThinker]

27 minutes ago, Tailhappy said:

Have you ever tested your car against sat nav value? What was the offset?

I've checked a number of my cars (3x Octy mk2, 1x Octy mk1, 2x Citroen Xantia) against Google[tm] maps (including the speed display before it was provided by maps) and have found around 10% over-read on the car speedos.  I did have plans to set up a dash cam inside the car to record both speedo and satnav but the project died as I couldn't find anywhere non-destructive to mount the camera to see both displays.  I'm sure there's an app out there somewhere that will do the magic on a smartphone ...

 

BTW I did also carry out speedo checks pre-Google by holding a steady indicated speed on the motorway whilst my passenger used a stopwatch to time the mile marker posts.  I can't find my notes from those experiments (some on two wheels) but IIRC the 10% figure was still about right.

Edited 20 April, 2023 by MikeTheThinker

[PetrolDave]

7 hours ago, Tailhappy said:

Have you ever tested your car against sat nav value? What was the offset? While sat nav isn't 100% reliable it does match the roadside speed displays which are pretty accurate.

Citroen C1 on OE tyres +7%

Citroen C1 on larger 16" wheels +1%

 

Audi RS4 on OE tyres +5%

 

Skoda Octavia 1.4TSI Combi on OE 17" wheels +5%

Skoda Octavia 1.8TSI Hatch on OE 18" wheels +4%

 

All against Road Angel GPS

 

[MikeTheThinker]

6 minutes ago, PetrolDave said:

Citroen C1 on OE tyres +7%

Citroen C1 on larger 16" wheels +1%

 

Audi RS4 on OE tyres +5%

 

Skoda Octavia 1.4TSI Combi on OE 17" wheels +5%

Skoda Octavia 1.8TSI Hatch on OE 18" wheels +4%

 

All against Road Angel GPS

 

So that puts my qualitative reports in their place!

 

[Tailhappy]

8 minutes ago, PetrolDave said:

Citroen C1 on OE tyres +7%

Citroen C1 on larger 16" wheels +1%

 

Audi RS4 on OE tyres +5%

 

Skoda Octavia 1.4TSI Combi on OE 17" wheels +5%

Skoda Octavia 1.8TSI Hatch on OE 18" wheels +4%

 

All against Road Angel GPS

 

5% is OK. Better than 10! My Fabia used to over read (75 when doing 70) but with larger tyres it now reads correctly. But the miles travelled is down a bit instead. 

[PetrolDave]

14 hours ago, Tailhappy said:

5% is OK. Better than 10! My Fabia used to over read (75 when doing 70) but with larger tyres it now reads correctly. But the miles travelled is down a bit instead. 

The legal requirement is to always overread but by no more than 10%, so manufacturers try to get a vehicle to overread by around 5-7% when on brand new OE size tyres to allow for wear and tolerances.

Edited 21 April, 2023 by PetrolDave

[Tilt]

On the occasions that I have checked versus Tom Tom Go520 .........

 

Speedometer always seemed to read about 3mph over what the sat nav indicated. Higher or lowering the vehicle speed made no difference .... generally 3mph.

 

[Tailhappy]

On 29/04/2023 at 20:37, Tilt said:

On the occasions that I have checked versus Tom Tom Go520 .........

 

Speedometer always seemed to read about 3mph over what the sat nav indicated. Higher or lowering the vehicle speed made no difference .... generally 3mph.

 

Think 3 mph is about the norm now. Just about right, gives you a little wiggle room as it were. 

[lol-lol]

38 minutes ago, Tailhappy said:

Think 3 mph is about the norm now. Just about right, gives you a little wiggle room as it were. 

 

Always worked on true speed plus 10% plus 2 mph so geniune 79 mph is nickable and I got nicked at 80 mph just rolling down the air balloon hill by one of the new very long distant speed measuring vans.  

 

Some police regions were going to do a smaller tolerance nicking.  Think how many drivers Bedford police would catch on the m1 if the camera was set to 73 ?

 

Been doing much more driving in Wales at driving at 20 mph in urban areas is tough, particularly in an Auto I find, less so in my Clio manual by sticking it in second gear.

[Ootohere]

For Scotland ignore the above.

Not Speedo accuracy, but the !0% + 2 mph & when NIP's might get issued from Safety Partnership Camera Vans, or Average Speed Cameras or being clocked by Police Scotland offers in vehicles or roadside with handheld devices.

Edited 1 May, 2023 by toot

[SteveTheElder]

Yesterday compared mine with Waze on iPhone/CarPlay at 60 & 70 mph. The car speedo was surprisingly close, never reading more than 2mph above Waze.

Octy 4 on part worn 205/55R17 tyres.

[lol-lol]

I do not know whether this muppet caught for speeding on the motorway at 69 mph was playing the percentages of 10% + 2mph,  or thereabouts,  and pushing his luck but I am seeing these 60 mph limits due to high pollution levels more and more.

 

https://www.thisisoxfordshire.co.uk/news/23500217.m1-speed-camera-catches-driver-69mph-motorway/

 

Whenever I see these speed limits, I alter my displays to digit deadout to be as accurate as possible for speed but if I am in the electric Zoe I think this speed limit should just be for the ICE cars and not for EVs which should be allowed to do the full National Speed Limit of 70 mph as we are not producing pollutant at anything like the shame level as ICE vehicles.

 

With number plate recognition combined with speed measuring should be do-able and could encourage the migration from ICE to EV. 

 

Edited 7 May, 2023 by lol-lol

[J.R.]

10 minutes ago, lol-lol said:

I think this speed limit should just be for the ICE cars and not for EVs which should be allowed to do the full National Speed Limit of 70 mph as we are not producing pollutant at anything like the shame level as ICE vehicles.

 

Freud has exposed your true colours Lol-Lol 😄

[J.R.]

For anyone who does the testing and finds that their speedo over-reads by 10% then be aware that if you have an indicated 130 km/h and  mentally deduct 10% (as easy as it is) to get the figure of 117 kp/h it will not be the true figure which would be 118.2 km/h so you are giving yourself a little extra bit of leeway which might just save you a fine.

 

For those who are not mathematicians or who do not have 5% or 10% accuracy then we just have to guess.

 

When I found a setting in my maxidot for winter tyres I naively thought it was for resetting the speedo accuracy, it isn't and I cant work out what its purpose is, it seems to be pointless, does anyone know?

[Warrior193]

12 minutes ago, J.R. said:

For anyone who does the testing and finds that their speedo over-reads by 10% then be aware that if you have an indicated 130 km/h and  mentally deduct 10% (as easy as it is) to get the figure of 117 kp/h it will not be the true figure which would be 118.2 km/h so you are giving yourself a little extra bit of leeway which might just save you a fine.

 

For those who are not mathematicians or who do not have 5% or 10% accuracy then we just have to guess.

 

When I found a setting in my maxidot for winter tyres I naively thought it was for resetting the speedo accuracy, it isn't and I cant work out what its purpose is, it seems to be pointless, does anyone know?

I believe it is for de-rating the maximum speed warning (if set) - winter tyres usually have a lower rating.

[lol-lol]

17 minutes ago, J.R. said:

 

Freud has exposed your true colours Lol-Lol 😄

 

Oh I feel the shame of driving an ICE car, even when it calls itself a hybrid, as I am always thinking of the pollution it is creating out of its tail pipe but it is difficult and I can blame the business system.  My employer, a French mega company, provides me with a fuel card for the hydrocarbon poison that my engine turns in to a deadly gaseous cocktail and we are, slowly, turning toward the whole EV thing, salary sacrifice as tax efficient scheme, chargers at our offices and somehow of compensating us from charging at home using the nuclear base load provided by EDF for much of Western Europe plus the rapidly increasing wind generation of course.  

 

Anybody who cares for their fellow man would advocate EVs to reduce the massive death toll created by ICE vehicles, closely followed by our home gas boilers I suspect. 

 

If the other giant French transport company, CMA, who also own CEVA, does in fact buy my employer then hopefully we will surge forward with more green measures, ships running on hydrogen and only EVs on the road, or some hydrogen if they can make it work and then we can only worry about the full National Speed limits without them being reduced because levels of pollution have hit dangerous level.

 

 Well done France, and Denmark and Norway, for producing so much surplus electricity they can sell it to other European countries during the night so we can run our EVs for a couple of pence a mile and hopefully keep on going at the NSL levels rather than being slowed down as we are polluting so much !!  

 

https://www.gridwatch.templar.co.uk/france/

 

[Ootohere]

@J.R. In Winter Tyres you are setting a Max Speed Limit Warning that you want,  + or - MPH or KMPH. 

You have the other Speed Warning that you can set as well.

 

In some countries you need that or the sticker for you or other drivers showing the Winter Tyre Max Limit. 

 

Edited 7 May, 2023 by toot