# Good read on speedometer error in C&D, April 2002



## jeffmc (Dec 20, 2001)

*Re: hey nate.....*

You can go to your avg. mph screen and reset it and it will give you your current speed.



DougDogs said:


> *is that photo touched up in photoshop????
> 
> if not, how does the OBC display current MPH ?????
> 
> I do not have that display on my OBC  *


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## glaws (Feb 21, 2002)

Hmm. didn't know that. Thanks for the tip.


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## doubletee (Dec 20, 2001)

Spiderm0n said:


> *
> 
> They make it look like you are going faster then you actually are. According to C&D tests, on the average BMW you have to set your speedometer to 73.18 MPH to actually be going 70 MPH *


Using a GPS, that's almost exactly what my results were, at any speed. Even at a speedometer-indicated 35 mph, the GPS said 31. Seemed to remain overstated by 4 mph all the way up to 100 mph.

I've just decided to subtract 4 mph from the speedometer reading when I'm trying to decide how fast I'm really going.


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## joe325i (Mar 10, 2002)

*OBC speed in real-time*

It's also possible to display current speed in real-time, updating on the OBC (without using that "reset the Avg. MPH" trick)

Does anyone have details or directions on how to do this? (It involves entering some undocumented modes, right?)

Thanks

Joe


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## Nick325xiT 5spd (Dec 24, 2001)

Honestly, I'm happy to have the speedometer read a little high. It gives me a touch more breathing room.
Nick


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## Desertnate (Mar 11, 2002)

Apperently BMW motorcycles suffer from this problem as well. I was flipping through a UK motorcycle magazine called "Bike" at the barbershop this weekend, and they ranked BMW's speedo's accuracy as the worst in the field of 4 or 5 different makers they tested. Don't remember how far off they said they were, but it was significant.


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## glaws (Feb 21, 2002)

Tried the OBC trick. At a steady 100 the OBC read 97.3. So 2.7% optimistic. Not b ad. Will try at higher speeds when I get the chance.


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## Spiderm0n (Dec 19, 2001)

Does it really matter if you are going 112 mph or 115 mph? I doubt the cops will be interested in the inaccuracy of the BMW speedometer at that speed! :yikes:


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## Cal (Jan 4, 2002)

glaws said:


> *Tried the OBC trick. At a steady 100 the OBC read 97.3. So 2.7% optimistic. Not b ad. Will try at higher speeds when I get the chance. *


I've tried mine numerous times at different speeds, but my results were different. At 100 mph indicated, my OBC read 101. At 80mph, it read 81.4mph. However, at 30 mph, my OBC read 30mph. It's so weird. Looks like my OBC is even more optimistic, especially if this BMW speedo thing is true. The C&D article stated that the OBC was correct, and the speedo was off.:dunno:


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## kryptonite (Feb 15, 2002)

i think the display current speed trick on the OBC is press 10 and 100( or 1000? ... leftmost two buttons, whatever they are) at the same time. then pick a number for the function you want. then hit check to exit. it's somewhere online...idon't remember the details


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## jpherit (Dec 22, 2001)

*My speedo is very accurate*

I just got the Delorme software and GPS to use in the car with my laptop. The Delorme Software displays the actual vehicle speed as determind by the GPS positions and time.

I have found that my low mileage 325i speedo is remarkably accurate. At 65 mph indicated on the speedo, the gps says 63.5 to 64. Even when the speedo indicated 80 the gps said 78 to 79.

In contrast, when my 90 Acura Integra speedo says 65 the gps says 59-60. The tires have about 25,000 mi on them so wear is an issue.

:thumb:


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## DrBimmer (Dec 23, 2001)

I tested it today and I was dead on. My speedometer said 85 and so did the OBC. I don't know what it is.... Why does everyone get different results?


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## JetBlack330i (Feb 8, 2002)

*Re: My speedo is very accurate*



jpherit said:


> *I just got the Delorme software and GPS to use in the car with my laptop. The Delorme Software displays the actual vehicle speed as determind by the GPS positions and time.
> 
> :thumb: *


If I may ask... How do you secure the laptop inside your car?
I'm also thinking in getting the Delorme GPS, but haven't figured out the laptop install issue.


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## JetBlack330i (Feb 8, 2002)

Cal said:


> *
> 
> I've tried mine numerous times at different speeds, but my results were different. At 100 mph indicated, my OBC read 101. At 80mph, it read 81.4mph. However, at 30 mph, my OBC read 30mph. It's so weird. Looks like my OBC is even more optimistic, especially if this BMW speedo thing is true. The C&D article stated that the OBC was correct, and the speedo was off.:dunno: *


I suspect there is only one sensor in the entire car and all readouts are fed from the same data.
I wouldn't be surprised if that sensor were something as simple as a wheel revolution counter, just like those on bicycle speedometers (you know... the little magnet on the wheel and a little sensor that sends a tick each time the magnet passes by...)
Given the number of revolutions per time interval, and given a known tire diameter, one can easily get mileage travelled, current and average speeds, etc. 
That would explain why putting different tire diameters offsets the speedometer.
If you assume the above, then your digital (OBC) and analog (speedometer needle) readouts must be the same. In your case, the difference could be easily explained if the needle was assembled slightly off (~1 mph) from the ideal position. I suspect that at 30mph your needle was actually registering ~29.

BTW, the OBC readout is an indication of your current speed only at the second immediately after you reset it. The readout at the next second is an average of the previous 2 seconds, and so on.
Even if you use the cruise control, I don't see how one readout can validate the accuracy of the other, which is what this thread is about.
For that you must use a second source of speed sensing, such as a GPS or good old mile marker counting and a timer.


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