Speedo and CC discrepency

[mr. shr]

Hi All,

Has anyone else noticed a difference between their speedo reading and the cruise control set speed?

For exmaple, if I set the CC when the speedo (both analogue and digital) is reading 70mph, the CC display will say "cruise control 68 mph"

So if I push the CC stalk up twice, the cruise control goes up to 70 mph, but the actual speedo (both ana and digi) will now read about 72 mph.

There seems to be a 1 or 2 mph difference at various speeds.

Anyone else noticed this, or should I be concerned?

 

[Spin_bowler]

Mine's exactly the same.

I've always relied on the fact that most speedo's read faster than actual speed to keep me (relatively) honest on the motorway. If you use a GPS speedo to calibrate in my experience they always read lower than the speed indicated by the car. Although when I did this in my S-Class the difference was only about 2 MPH at 90MPH which is more accurate than other cars I've owned/driven.

Not sure why the two sources in the car would be different though. I'm sure someone will be along with an explanation but in the meantime, should you be concerned? I think you can sleep safe tonight.

 

[mr. shr]

If you use a GPS speedo to calibrate in my experience they always read lower than the speed indicated by the car. Although when I did this in my S-Class the difference was only about 2 MPH at 90MPH which is more accurate than other cars I've owned/driven.

Do you mean with a Tom Tom or something?
You can't get the speed displayed on COMAND can you?

 

[Spin_bowler]

Yep with a Tom-Tom or handheld GPS. COMAND not that I know of, but I'm no expert. I know that you can get the roadspeed on the heating/ventilation display in the diagnostic menu (in km/h) but I assume that is from the same feed as either the cruise or the speedo (if indeed, they're different in the first place).

 

[whitenemesis]

Hi All,

Has anyone else noticed a difference between their speedo reading and the cruise control set speed?

For exmaple, if I set the CC when the speedo (both analogue and digital) is reading 70mph, the CC display will say "cruise control 68 mph"

So if I push the CC stalk up twice, the cruise control goes up to 70 mph, but the actual speedo (both ana and digi) will now read about 72 mph.

There seems to be a 1 or 2 mph difference at various speeds.

Anyone else noticed this, or should I be concerned?

You say "about", how "about" do they read? It seem an insignificant difference, certainly not worth worrying about

 

[mr. shr]

Yep with a Tom-Tom or handheld GPS. COMAND not that I know of, but I'm no expert. I know that you can get the roadspeed on the heating/ventilation display in the diagnostic menu (in km/h) but I assume that is from the same feed as either the cruise or the speedo (if indeed, they're different in the first place).

I would have thought the speedo and CC would be all drived from the same source for the ESP to work correctly, but I'm no expert either.

Reassuring to know yours does the same though.

 

[mr. shr]

You say "about", how "about" do they read? It seem an insignificant difference, certainly not worth worrying about

Yes, I'm only talking about a couple of mph difference but I thought it odd.
I thought the car used the same source for systems.

I haven't tried setting the speed limiter to, say 70mph, and observed the actual speedo reading on cut out. Wil try that I guess.

But yes, nothing worth worrying about, I just wondered if anyone else had experienced it on their cars?

 

[whitenemesis]

I would have thought the speedo and CC would be all drived from the same source for the ESP to work correctly, but I'm no expert either.

Reassuring to know yours does the same though.

What would ESP have to do with CC or Speedo? I thought it's all to do with differences between wheel speeds and differences between yaw angle and steering input

 

[MBManInKen]

On my W220, they are exactly the same, which is what you would expect. Surely both displays use the same data source?

 

[mr. shr]

What would ESP have to do with CC or Speedo? I thought it's all to do with differences between wheel speeds and differences between yaw angle and steering input

Well like I say, I'm no expert. I'm just assuming the ESP reads signals from the speed pulse sensor, as does the speedo system and thought they probably are inter-related somehow??!!

 

[mr. shr]

On my W220, they are exactly the same, which is what you would expect. Surely both displays use the same data source?

Well that's what I thought, hence I asked.

Seems I WILL have trouble sleeping tonight.

 

[Shude]

I doubt it has anything to do with it but the speed pulse is measured in kph and then converted into mph by the instrument cluster.

I think the cruise control increments by levels of kph, not mph, so sometimes it might look like it jumps a bit if you're reading a digital display, it has no way to display decimal places perhaps?

 

[mr. shr]

I doubt it has anything to do with it but the speed pulse is measured in kph and then converted into mph by the instrument cluster.

I think the cruise control increments by levels of kph, not mph, so sometimes it might look like it jumps a bit if you're reading a digital display, it has no way to display decimal places perhaps?

Interesting point, but MBMANINKEN says his readings are all identical.

 

[Shude]

Interesting point, but MBMANINKEN says his readings are all identical.

Do you have the same model as him? How about this: Do you have the same wheels and tyres as him?

I love that COMAND asks whether your tyres are new or used on the calibration menu

 

[whitenemesis]

Do you have the same model as him? How about this: Do you have the same wheels and tyres as him?

I love that COMAND asks whether your tyres are new or used on the calibration menu

Does it?? Don't recall mine asking! When does that happend?

 

[mr. shr]

Do you have the same model as him? How about this: Do you have the same wheels and tyres as him?

I love that COMAND asks whether your tyres are new or used on the calibration menu

Well we have similar models (although his is much better).
Not sure about his tyre sizes.

 

[mr. shr]

Does it?? Don't recall mine asking! When does that happend?

Well it doesn't actually ask you. You have to tell it when you have new tyres fitted.

 

[Shude]

Does it?? Don't recall mine asking! When does that happend?

My COMAND unit is a DX, CD-type. Under the calibration menu you are asked to enter your tyre sizes and whether the tyres are new or used. It uses the speed pulse to such accuracy

 

[whitenemesis]

My COMAND unit is a DX, CD-type. Under the calibration menu you are asked to enter your tyre sizes and whether the tyres are new or used. It uses the speed pulse to such accuracy

mine is DVD, I don't recall any such questions?

 

[Shude]

mine is DVD, I don't recall any such questions?

I'd take pics but my car is 30 miles away at Ian Walker's place being made roadworthy again after being declared an MOT failure (exhaust hanger rattles and brake pipe has surface corrosion!).

My point about wheel and tyre sizes is valid though. You'd think that both functions would use the same input but it's entirely possible that they don't. The basic functions of the instrument cluster probably come from traditional sources but advanced functions might get their data from elsewhere. If you plug a modern cluster into an older car the speedo still works but other things do not - I have tried this .