Are BMW speedos inaccurate?

[treone 648]

Driving my '96 e31 and '06 e60 back to back over the weekend and I've noticed something interesting - when going along the same stretch of road that has a 'Your Speed' electronic board (I realise its indicated and probably not accurate), travelling at the speed limit, my speedo says 50km/h and the sign indicates 45km/h. Hmmm one off error?, well it is exactly the same for both.

Thinking that the sign has a variance, go the same route in my HSV and viola - speedo and sign both the same at 50km/h.

This might explain getting past by everything in the M5 when cruise control on at '100' when on the motorway haha 🤣

As the BMW's that are a couple of generations apart technology wise - could it be coincidence or is it a BMW thing (both are standard/factory set-up - wheels/tyres etc)? Anyone one have similar variances? Any fixes other than setting cruise 8km/h higher to keep up lol? 

[Goose 44]

I have noticed a similar thing with the “your speed” thing on the Waipu straits between my e39 and Landrover Disco. However checked it with a gps / speedo app on phone and both seemed to show 100kph on gps and speedo, or very close to. 
potentially something to do with vehicle shape as those signs use radar / lidar to indicate speed. That’s my best guess?

[Eagle 1664]

The average car reads around 5km over actual GPS speed in my experience, most of BMW ive owned have as well iirc except my old E30 which read 10-11kph too high. Current cars are both +\- 1km though.

Probably some correction value programmed into the instrument cluster.

[Gaz 1061]

All vehicle speedos will read slightly high for legal reasons

[aja540i 1906]

My M5 and my Leaf both show about 7 kph low when compared to Gooble maps GPS speed reading, you could try getting a Police issuecalibrated cluster but I don't like your chances, some of the programming software also shows the correction value for the speed displayed and lets you adjust it.

[Spinner99 55]

The car lies to you  I've noticed if I'm driving along with the instrument cluster saying 50hm/h, ISTA reports the speed as lower (e.g. ~47km/h).  My conclusion was the car knows exactly how fast it's going, but it inflates that a little on the instrument cluster for reasons.  For me 53km/h on the instrument clustser = 50km/h IRL.

I suspect this is something to do with variances in tyre diameters etc and them wanting to avoid the opposite where you're going faster IRL than the instrument cluster says, as that could open them up to having to pay for speeding tickets etc.  One of the giveaways for me is the RPM, I'm making up numbers at this point for illustrative purposes, but if the car is sitting on 1950 RPM and tells you it's doing 50km/h, it's likely that 2000rpm is probably a true 50km/h.

[treone 648]

Thanks for insights! Forgot about the built in 5km tolerance lol. At least BMW are consistent in my = two 90's era and one '06 reading exactly the same. Looks like its 2 options: cruise at 100 and watch the world go by or cruise at 108 and just keep pace (at Wellington at least 😎) ... or option 3... M-mode haha 

Will give GPS speed checking a go

Edited April 7 by treone

[Bruski 35]

2 hours ago, Eagle said:

The average car reads around 5km over actual GPS speed in my experience, most of BMW ive owned have as well iirc except my old E30 which read 10-11kph too high. Current cars are both +\- 1km though.

Probably some correction value programmed into the instrument cluster.

I'd agree with that.  The newer Audi is definitely +1, maybe 2kph difference.  

The y2k bimmer however, at a vmax200 event in the UK many, many moons ago, the dial was showing a somewhat shaky 163mph but clocked at 160mph on the timing beam, so 5mph over the stated limit in the manual, but still, 3mph/4.8kph off being 'accurate', at least its consistent all the way through the speedo range.  That shows something for 1990s engineering and technology.

[M3AN 4016]

I'm a little concerned this is even a question. All drivers should know these two things:

1. Your speedo is wrong
2. Live roadside speed indicators are wrong

Always have been, always will be.

And yes, probably the reason cars are banking up behind you when you think you're doing the limit.

 

[treone 648]

Its a genuine question and sometimes it good to be reminded of these things.

I’d say, the ‘fact’ that they are wrong is more confusing than concerning. My guess is majority of drivers don’t actually realise their speedo is out and/or don’t give a hoot either - just observe the various driving styles/behaviours (slow, fast and dickhead) out in the wild 😁. 

At this stage of life, quite enjoy driving at the speed limit knowing that ‘I can’ if I ever wanted to 😎. Years gone past it was completely the opposite.

Now that I know (again), all that’s needed is to make the appropriate adjustment 😎 … Time to engage ‘Driving Miss Daisy’ mode haha 🤣

Edited April 7 by treone

[Spinner99 55]

FYI There are a couple of coding options i found to fix it, I made them but I didn't see any difference but it might work for someone else?  If it does please let me know.

Both in the KOMBI module.  Changing from aktiv to nicht_aktiv is supposed to show the true car speed.

BC_DIGITAL_V_KORREKTUR

BC_V_KORREKTUR

[Jun 233]

On 4/8/2024 at 5:42 AM, treone said:

Its a genuine question and sometimes it good to be reminded of these things.

I’d say, the ‘fact’ that they are wrong is more confusing than concerning. My guess is majority of drivers don’t actually realise their speedo is out and/or don’t give a hoot either - just observe the various driving styles/behaviours (slow, fast and dickhead) out in the wild 😁. 

At this stage of life, quite enjoy driving at the speed limit knowing that ‘I can’ if I ever wanted to 😎. Years gone past it was completely the opposite.

Now that I know (again), all that’s needed is to make the appropriate adjustment 😎 … Time to engage ‘Driving Miss Daisy’ mode haha 🤣

Here is the long answer https://www.aa.co.nz/cars/motoring-blog/why-does-my-speedo-read-higher-than-road-side-speed-monitors/

it’s not an issue for cars I put bigger tires like Prado or Cayenne, taller tires will roll longer but you may over speed even if your speedometer is under speed limit. I find the Tesla to be 1 kmh under reporting only just comply w/ this weird rule.