[dkexige]

For the most part I just drive relative to everyone else's speed accordingly.

[Antetokounmpo]

It's 5% off as many have said... I have the "true speed" coded into my dash with my mpg, range, etc. and it matches perfectly with my gps and police radar things on the side of the road.

[bmorebblick]

I've decided I'm going to get into this coding nonsense.... anyone know where a cheap and capable cable can be purchased?

The thread I posted earlier did say though, that you can't actually code out the skew in the needle, just that the work around was to add that V= thinger.

Was that tough at all for those who did it?

I just cant stand being in the "flow" of traffic, and my car telling me I'm going 80mph....There's no way that jalopy next to me can even go that fast!

[fazman]

Originally Posted by wilbur_the_goose

I solved the issue by having the coding done to enable display of the "v" (velocity) on the dash where the MPG/Average speed is listed.

If you don't change the BMW skew setting... it will still show the V=real speed + BMW skew

[fazman]

Originally Posted by Liang

Does it actually the actual speed even if your running wheels/tires at different sizing than factory?

If you change your wheels to a different overall rolling diameter, then you are going to throw off a lot of stuff. The car expects the rolling diameter to be the stock height. Usually because of wheel well clearances you can't change it too much tho. But it would be nice if cars actually used the built in GPS to "auto correct" the land speed or a way to code the the rim size and tire size for each wheel into the car so it can help compensate.

[floydarogers]

Originally Posted by Liang

Does it actually the actual speed even if your running wheels/tires at different sizing than factory?

No (as already stated). My winter tires are 225/45R18 rather than the OEM 225/40R18. They "corrected" the speedo error from the 3-4 mph range (70 mph) to only 1 mph fast.

[wilbur_the_goose]

fazman - the skew factor was corrected by my coder.

[bufasion]

[QUOTE=Rossland;14460631]

Originally Posted by bmorebblick

Son of a -_-

So annoying... I wish they would just give me an accurate read out.

I agree. I have never been able to understand why the speedometer in my Chev truck can be right on, yet my BMW is so far off.

I drive a lot of rental cars. Out of hundreds of rentals ranging from Kia to Lincoln I've NEVER found any car that has a speedo as far off as BMW. For a while I would take pictures of my Garmin GPS right next to the rental's speedometer. They are always within 1 mph.

If there's a reason for me not to get a BMW again this would be it. Anybody know if M-B or Audi has the same problem?

[johmei]

Originally Posted by Liang

Does it actually the actual speed even if your running wheels/tires at different sizing than factory?

The car cannot correct for tire size. Get smaller diameter tires and you will get a higher speed reading. Your cars odometer will also run quicker making you stack up miles a little faster.

At least in Sweden it is not allowed to have more than 5% difference (+/-) in roll circumference from OEM specification.

[floydarogers]

Originally Posted by bufasion

Anybody know if M-B or Audi has the same problem?

Again: it's not a "problem" if it's by design. Yes, all EU cars (including our Porsche) have the same (or similar) design. Although I remember one Audi owner claiming his speedo is dead accurate.

[Blackhawk36]

Originally Posted by johmei

The car cannot correct for tire size. Get smaller diameter tires and you will get a higher speed reading. Your cars odometer will also run quicker making you stack up miles a little faster.

At least in Sweden it is not allowed to have more than 5% difference (+/-) in roll circumference from OEM specification.

I believe in Germany it is not +/- it is just +5% -0%. Basically manufacturers are not permitted to spec a speedometer where a driver can put on larger (within spec) tires and "accidentally" run afoul of the speed limit.

[Eagle1oh7]

Anybody running large wheels? That contributes to a loss too.

[pk_]

My W204 MB C350 was pretty bang on.. Didnt even take notice in my e92.....after the water pump gets replaced by BMW this weekend ill be sure to look out for it...

[alexwhittemore]

Everybody have a good read:
This wikipedia entry is very enlightning:
http://en.wikipedia.org/wiki/Speedometer#Error

In short, when you sum up all the sources of error between the road and the needle (including tire under/over inflation, actual installed tire diameter which may be different from factory fitted wheels, actual measurement error inherent to the system used and its construction tolerances, etc), the overall measurement uncertainty is pretty significant.

Various regulations govern how this uncertainty must be handled, but in the EU in particular:

-The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
-The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

The same is true of radar guns used in enforcement: if the argument could be made that the gun's tolerance extends 5% below the reported reading, and therefore the gun could be overestimating by 5%, the ticket would get thrown out every time. This is where your 5-7 MPH grace spread comes from. This is why when you pass those speed signs on the street, it usually reads lower even than what your GPS says you're going.

tl;dr, every system has measurement uncertainty and therefore, by various laws depending on jurisdiction, a car may be legally required to overreport your speed.

[alexwhittemore]

Originally Posted by Eagle1oh7

Anybody running large wheels? That contributes to a loss too.

If your running radius is larger than factory spec, the speedo will UNDERreport your speed. Larger wheel means lower RPM for the same linear speed.

And this is sort of the point. If you install larger wheels on the car, per legalese it still shouldn't ever report that you're going slower than you really are.

[atinybug]

Originally Posted by alexwhittemore

Everybody have a good read:
This wikipedia entry is very enlightning:
http://en.wikipedia.org/wiki/Speedometer#Error

In short, when you sum up all the sources of error between the road and the needle (including tire under/over inflation, actual installed tire diameter which may be different from factory fitted wheels, actual measurement error inherent to the system used and its construction tolerances, etc), the overall measurement uncertainty is pretty significant.

Various regulations govern how this uncertainty must be handled, but in the EU in particular:


The same is true of radar guns used in enforcement: if the argument could be made that the gun's tolerance extends 5% below the reported reading, and therefore the gun could be overestimating by 5%, the ticket would get thrown out every time. This is where your 5-7 MPH grace spread comes from. This is why when you pass those speed signs on the street, it usually reads lower even than what your GPS says you're going.

tl;dr, every system has measurement uncertainty and therefore, by various laws depending on jurisdiction, a car may be legally required to overreport your speed.

This makes a lot of sense. A while ago I had a e90 328i as a loaner from the dealer, and that thing's speedo was so far off I could notice it without a GPS. I couldn't tell exactly how far off, but cruising around left lane on the highway following the flow of traffic, the speedo would indicate nearly 90mph. And going by the speed sign near my house, speedo would indicate 35, sign would say 25.

But that was the only car that I've noticed that's been off so much. My old E46 felt somewhat accurate (going with flow of traffic speedo would say 82-85), same with my new E92.

I remember seeing an article a long time ago that compared how accurate the speedos of different manufacturers were, and Chevys ended up being the most accurate or something.

Found it. http://www.caranddriver.com/features...ometer-scandal

[SSW]

Based on speed readings made with my Cobb AP3, the speedo in my 335i runs about 5 mph "fast" at 70+ mph.

I've read threads that say that there is no software fix for this and that you just have to accept it. HOWEVER, I use a SuperChips tuner in my Ford F250 which has a speed adjustment function that allows you to correct the speedometer reading in the truck based on the measured tire diameter.

So, if the SuperChips tuner can make a speed adjustment for my truck, why can't anyone (who has the necessary skills) design a simple program to make the same adjustment in our BMW's?

[GB-335xiCoupe]

Originally Posted by dkexige

For the most part I just drive relative to everyone else's speed accordingly.

For the most part, I just pass everyone. Usually quickly. Even cops.

[cabimmer]

^what he said.

[335BOY]

Originally Posted by bmorebblick

Son of a -_-

So annoying... I wish they would just give me an accurate read out.

Edit -
To anyone who reads this post wondering the same thing, I found this thread:
http://www.e90post.com/forums/showthread.php?t=740977

There seems to be an alternative method of reading speed through coding, but other than that the only fix is to physically adjust the needle =(

WHy does it matter?? It saves those that don't know lots of speeding tickets. How could this bother anyone?

[SSW]

Originally Posted by 335BOY

WHy does it matter?? It saves those that don't know lots of speeding tickets. How could this bother anyone?

It doesn't matter in a practical sense, as long as you know how much "off" your speedo is, BUT it "matters" to me (and others) who prefer their gauges be accurate and who would rather not "guess" what their speed actually is.

It "could" also matter legally if you are caught on radar driving at a certain speed above the limit because you inaccurately "guessed" what your speed was. For example, if you are driving at "70" on the speedo in a 65 mph zone in the expectation that the error is 5 mph but the error is actually only 3-4 mph, you could get a ticket for driving at 66-67 mph.

It would be a chickensh*t ticket but you could still get one for that nonetheless. If you're using a digital speedo and it's accurate, there would be no such issue because if it shows you driving at 65 that's what it is and you don't need to worry about guessing what the degree of error in the speedo is at all.

[335BOY]

Originally Posted by SSW

It doesn't matter in a practical sense, as long as you know how much "off" your speedo is, BUT it "matters" to me (and others) who prefer their gauges be accurate and who would rather not "guess" what their speed actually is.

It "could" also matter legally if you are caught on radar driving at a certain speed above the limit because you inaccurately "guessed" what your speed was. For example, if you are driving at "70" on the speedo in a 65 mph zone in the expectation that the error is 5 mph but the error is actually only 3-4 mph, you could get a ticket for driving at 66-67 mph.

It would be a chickensh*t ticket but you could still get one for that nonetheless. If you're using a digital speedo and it's accurate, there would be no such issue because if it shows you driving at 65 that's what it is and you don't need to worry about guessing what the degree of error in the speedo is at all.

Its 3 or 4 MPH. Simple. Everyone knows and /or can check with GPS app on smartphone. On one hand I understand what you are saying but on the other , what is 3 or 4 mph compared to 5 as you mention above. Really?