Speedometer Oddity

pmeloche

Senior member
 MontrĂ©al, Qc
I had my Z4MC detailed at Reeves BMW in Tampa today and they gave me an automatic 328i for the day. Overall, a most excellent car but not a Z4MC. :) I kept staring at the speedometer and could help myself but finding it odd. Then I realized that the dial was in mph and the inner dial in kph. How bizarre is that? The canadian-based BMWs have only a metric dial for a speedometer.

Why would that be? Since Canada and the U.S. of A are neighbours with different measurement systems you would think that the speedometers would show both scales, with the main scale being the scale of the base country. IMO, it is more likely that a canadian will travel to the U.S. of A than the opposite. Who in their right mind would want to willingly drive in Canada? :lol:

I wonder what is the rationale for BMW to do that if they have even considered it other than saving the cost of a different dial for a small market like Canada? I would imagine that the speedometers in Europe are all metric-only except for the U.K-bound cars who would have an Imperial-Metric speedometer, which would explain the speedometer on the U.S.-bound cars. For Canada, you would need an additional speedometer which would be metric-Imperial, therefore we get the metric-only speedometer. A minor annoyance in any case. :x
 
You were in Tampa so you had the US dial. MPH large on outer circumference, kph smaller on the inner. Speedos don't use gears off the tranny anymore-it's done with the engine management chip so you all you need is a different face of the speedometer--cheap to print after you've established where the marks go to correspond to which scale you're using.
 
I think its purely because the amount of countries that use Imperial distances are vastly in the minority these days.

In fact I can only think of the UK and USA!! Come on guys lets stick together on this one, at least Knots are still used worldwide :thumbsup:
 
Smokin said:
You were in Tampa so you had the US dial. MPH large on outer circumference, kph smaller on the inner. Speedos don't use gears off the tranny anymore-it's done with the engine management chip so you all you need is a different face of the speedometer--cheap to print after you've established where the marks go to correspond to which scale you're using.

Are you sure the speedo is connected to the engine management chip??? I could have sworn otherwise, as the speedo will display the correct speed if you put it in neutral when driving. Same thing goes if you roll down a hill without the ignition fully on (just position 2 (?) to engange the electrical system? -Not that the gas price has deterred me from turning on the engine :wink:
 
On the Z4, there is a wheel speed sensor for each wheel which feeds the CAN network, which feeds the DME so the DME can process vehicle speed, which in turn feeds the instrument cluster to power the electric motor to turn the speed pointer. It also allows the DME to determine transmission mode (automatics) and feeds the Dynamic Stability Control module for the DSC.

On some vehicles, my Ford truck for this example, the speedometer gets it's signal from the VSS (Vehicle Speed Sensor) which is on the output shaft of the tranny.

The VSS sends out pulses corresponding to the rpm of the drive shaft, which in turn is calibrated to a particular speed and is fed to the speedometer motor which turns the dial.

Each manufacturer has their own way of doing things. Gone are the days of a cable to the instrument cluster...
 
You mean I couldn't just jack up my rear wheels and put the gear in reverse and rolll back the mileage? :o :D
 
Smokin said:
all you need is a different face of the speedometer--cheap to print after you've established where the marks go to correspond to which scale you're using.

You've got to be kidding! :?
 
Smokin said:
Well, relatively cheap--it's a photoresist printing method

Are you suggesting to add over the current dial an inner dial in Imperial measure? That's an interesting idea but I don't think that is legal. The canadian will not like that and I suspect the U.S. DOT also will frown upon that.
 
Shipkiller said:
On the Z4, there is a wheel speed sensor for each wheel which feeds the CAN network, which feeds the DME so the DME can process vehicle speed, which in turn feeds the instrument cluster to power the electric motor to turn the speed pointer. It also allows the DME to determine transmission mode (automatics) and feeds the Dynamic Stability Control module for the DSC.

So you're saying the Z4 doesn't have the speed sensor in the differential? After you mentioned it I looked it up on RealOEM and couldn't find it...

The Z3 does and I never understood why the speedo uses the diff signal, while the OBC uses the ABS wheel sensor... Why do they have two separate sources for the same reading?

Another thing I've noticed and it's interesting:
The speedo is always higher than the actual speed (I think it's a DOT requirement that under no circumstances it can show a speed lower than the actual... However you define the vehicle speed...) while the OBC is always correct, or at least it matches the speed reported by my GPS and a simple "distance over time" (I get bored on long trips, so I gotta keep myself busy somehow... :oops:) ... The odometer is also accurate (the distance reported under trip matches what calculated by Google maps)... So how does it work: both the speedo and the odometer take the reading from the diff, which is "correct", but the speedo "shows" a higher speed (as simple as having the scale tweaked in such a way... now that I think about it the angle between 0 and 10 is smaller than every other... could this be it?), or the speedo gets it from a "wrong" diff sensor, and the odometer from the "correct" ABS wheel sensor??? Is the diff speed used by anything else (eg ECU, cruise control, ABS)?

PS: I'm starting to loose my sleep over these hamletic dilemmas... :tumbleweed:
 
pmeloche said:
Smokin said:
Well, relatively cheap--it's a photoresist printing method

Are you suggesting to add over the current dial an inner dial in Imperial measure? That's an interesting idea but I don't think that is legal. The canadian will not like that and I suspect the U.S. DOT also will frown upon that.

No, I'm saying that the cost of BMW having different dials for different markets really is of no cost in the long run--once you have the formula for how far the car moves for one rotation of the tires and how fast it does so, you can call it any units you want on the dial. The printing of that dial is negligible in the grand scheme of things.
 
Zeta, I was confused also. Before I posted that last post, I went into the BMW electrical DVD and looked.

I am not sure which wheel they use, or they use all of them...
 
Shipkiller said:
Zeta, I was confused also. Before I posted that last post, I went into the BMW electrical DVD and looked.

I am not sure which wheel they use, or they use all of them...

LOL... If you're confused then it must be one of the most well guarded BMW electronic secrets... :driving: :driving: :driving:
 
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