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      10-07-2009, 11:59 AM   #1
BlueZ4AZ
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Speedometer Accuracy

My speedometer is off by about 8%. It reads 50 mph, when I am going 46 mph. I know this for sure, as I have driven by several roadside digital radar guns that flash your mph for you. Every single time I drive by one going 50 on my speedometer the radar reads 46.

Is anyone else having an issue with speedometer acuracy?
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      10-07-2009, 12:22 PM   #2
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i read somewhere, that bmw considers its speedometer "good" if it's within 10%. to me, i consider that total non-sense.
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      10-07-2009, 12:29 PM   #3
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Quote:
Originally Posted by BlueZ4AZ View Post
My speedometer is off by about 8%. It reads 50 mph, when I am going 46 mph. I know this for sure, as I have driven by several roadside digital radar guns that flash your mph for you. Every single time I drive by one going 50 on my speedometer the radar reads 46.

Is anyone else having an issue with speedometer acuracy?
That's unfortunately not unusual for BMW. My Z4 is much more accurate than my Z3 was, but is still about 3-4% low. Given that my Z3 was about 8-9%, I'm pleasantly surprised with only 3-4% for a BMW.

I tend to rely on either my new Escort Passport 9500ix with display set to mph, or my GPS if it's turned on.

Last edited by willysp; 10-07-2009 at 05:25 PM..
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      10-07-2009, 12:31 PM   #4
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Quote:
Originally Posted by mikeinaustin View Post
i read somewhere, that bmw considers its speedometer "good" if it's within 10%. to me, i consider that total non-sense.
Yes, I saw the BMW service bulletin and 10% is correct. To be precise, specs are -10% / +0%. And I agree it's ridiculous.
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      10-07-2009, 12:37 PM   #5
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This is a known issue for all cars and motorcycles from all companies.
The speedometer always overestimates your speed. I think it's a safety measure. Futhermore you must take into account that speed limits in some countries are very strict, so the car shows a little bit increased speed, just to ensure compliance with speed limits for the driver.
Let's say that the current speed limit is 100km/h. If I set my cruise speed to 100 km/h, Im absolutely sure that I will never violate the limit, since the actual speed of the car is less than 100 km/h. So no speed camera will ever catch me.
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      10-07-2009, 12:45 PM   #6
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actually my infiniti is dead accurate up to 70mph.
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      10-07-2009, 12:45 PM   #7
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I don't care what BMW says, they're fixing it.

The issue is that my total miles are being overstated by 8%. When I sell the car in a few years it will show 8% more miles on the odometer than it should have.

I plan to ask BMW if they are going to either fix the speedometer or re-emburse me for the difference in the sales price of the car, based in the fact it will have more miles on it than it should when I sell it.
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      10-07-2009, 03:37 PM   #8
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Mine reads about 2mph over at 70mph and between 1 and 2mph over at 30. My TomTom shows the accurate speed and I know to set my cruise at 72 to be on the speed limit
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      10-07-2009, 04:49 PM   #9
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Quote:
Originally Posted by jpapg View Post
This is a known issue for all cars and motorcycles from all companies.
The speedometer always overestimates your speed. I think it's a safety measure. Futhermore you must take into account that speed limits in some countries are very strict, so the car shows a little bit increased speed, just to ensure compliance with speed limits for the driver.
Let's say that the current speed limit is 100km/h. If I set my cruise speed to 100 km/h, Im absolutely sure that I will never violate the limit, since the actual speed of the car is less than 100 km/h. So no speed camera will ever catch me.
My girlfriend's Infinit EX35 is always exact, so was my Corvette, so this cannot be an issue with all cars from all companies.

The automobile has been around for 100+ years, we can send a man to the moon and study new rings around the planet Saturn now. So most certainly there must be a way for human beings to calibrate a speedometer correctly.

I don't much care what BMW sets as the acceptable parameter (+/- 10%). It's what's acceptable to me since I'm making the PAYMENTS. It may come down to how bad one decides to complain about it to the service dept, to get it fixed.
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      10-07-2009, 04:57 PM   #10
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Quote:
Originally Posted by jpapg View Post
This is a known issue for all cars and motorcycles from all companies.
BMW is well known for erring much more on the side of safety than other car companies. 1%-3% is normal for most car companies. 5%-10% isn't unusual for BMW.

Believe me - I've done the research, read the BMW bulletins, and owned many cars from multiple companies.

IMO, BMW takes it to an extreme.

Last edited by willysp; 10-07-2009 at 05:34 PM..
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      10-07-2009, 05:13 PM   #11
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Quote:
Originally Posted by BlueZ4AZ View Post
I don't care what BMW says, they're fixing it.

The issue is that my total miles are being overstated by 8%. When I sell the car in a few years it will show 8% more miles on the odometer than it should have.

I plan to ask BMW if they are going to either fix the speedometer or re-emburse me for the difference in the sales price of the car, based in the fact it will have more miles on it than it should when I sell it.
It doesn't affect the odometer - the odometer will be accurate. Also, the computer display will display the current speed correctly when you reset it, and will display average mph correctly. It's just the speedometer gauge on the dash that's (intentionally) wrong.

For BMW, speedo gauge variance of up to 10% + 2.4 mph is per design. Therefore, they won't fix something that isn't broken - if they try to adjust your speedometer, they run a risk of liability.

I don't like it either, and gnashed my teeth with my Z3. On several Z3 forums, there were reported discussions with a 3rd party company about adjusting it - but the liability issue killed it.

From a 1996 Service Bulletin:

SI1996-620296 SUBJECT:
Maximum Permissible Speedometer Error

MODELS:
All

General Information:
A certain amount of "speedometer advance" is necessary to compensate for negative tolerances in tire diameter, electronic controls, and other factors. This advance reduces the risk of the speedometer displaying a speed that is less than the vehicle's actual speed.

The amount of speedometer advance can be calculated by inputting a frequency (using the DIS Tester, "Kombi Test Schedule") that corresponds to a given speed and noting the speed that is indicated by the speedometer. The maximum permitted speedometer advance is 10% of the actual (input) speed plus 2.4 mph.

Example:

Actual Speed = 50 mph

50 mph x 10% = 5 mph; 5 mph + 2.4 mph = 7.4 mph

Therefore, the permissible displayed speed is 50 to 57.4 mph.

(Note: The displayed speed must never be less than the actual speed).

No repairs should be attempted if the speedometer advance is within 10% + 2.4 mph.

NOTE: The amount of speedometer advance has no effect on recording of accumulated mileage in the odometer display. The odometer records total mileage digitally and does not incorporate any "advance" tolerances.

Warranty Status:
Information Only.

Last edited by willysp; 10-07-2009 at 05:28 PM..
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      10-07-2009, 06:13 PM   #12
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[quote=willysp;5992904]....Also, the computer display will display the current speed correctly when you reset it, and will display average mph correctly. It's just the speedometer gauge on the dash that's (intentionally) wrong.

Good info. Thanks.

However, in the 2009 Z4, where is the digital speedometer? I read the owners manual and it said nothing about this, nor can I pull it up when toggling between, Avg MPH, Instant MPG, and Avg MPG on the digital display.
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      10-07-2009, 07:56 PM   #13
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[QUOTE=BlueZ4AZ;5993221]
Quote:
Originally Posted by willysp View Post
....Also, the computer display will display the current speed correctly when you reset it, and will display average mph correctly. It's just the speedometer gauge on the dash that's (intentionally) wrong.

Good info. Thanks.

However, in the 2009 Z4, where is the digital speedometer? I read the owners manual and it said nothing about this, nor can I pull it up when toggling between, Avg MPH, Instant MPG, and Avg MPG on the digital display.
When I said "the computer display will display the current speed correctly when you reset it", I meant "at the instant when you reset the average mph". So, not real useful, but was just making the point that the computer knows what your real speed is, therefore the odometer will be accurate.

I tend to use my Escort Passport 9500ix with display set to mph as my speedometer when I need to see a number I can trust.

BMW's approach is stupid, IMO. I drive faster that I would if the speedo were accurate - I know it's low, so I go faster to compensate. This increases the risk that I go too fast. I know for a fact, after looking at my GPS and my Passport, that the speedo inaccuracy causes me to go faster than I would if the speedo were accurate.
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      10-07-2009, 08:14 PM   #14
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[QUOTE=willysp;5993706]
Quote:
Originally Posted by BlueZ4AZ View Post

When I said "the computer display will display the current speed correctly when you reset it", I meant "at the instant when you reset the average mph". So, not real useful, but was just making the point that the computer knows what your real speed is, therefore the odometer will be accurate.

I tend to use my Escort Passport 9500ix with display set to mph as my speedometer when I need to see a number I can trust.

BMW's approach is stupid, IMO. I drive faster that I would if the speedo were accurate - I know it's low, so I go faster to compensate. This increases the risk that I go too fast. I know for a fact, after looking at my GPS and my Passport, that the speedo inaccuracy causes me to go faster than I would if the speedo were accurate.
I agree, I would be closer to the speed I want to be if the speedometer was accurate. I've had 6 BMWs. Most have been 1-3mph over the actual mph at 60 or 70 mph. A slightly taller can change the equation but I don't know how much larger a tire the Z4 can use.
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      10-07-2009, 10:26 PM   #15
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Two things come to mind:

1. What about 8-10% affect on warranty?

2. Computer adjustments for tire sizes?
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      10-08-2009, 08:53 AM   #16
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Quote:
Originally Posted by teagueAMX View Post
Two things come to mind:

1. What about 8-10% affect on warranty?

2. Computer adjustments for tire sizes?
What does #1 mean? If you're refering to odometer miles, please see the posts above. The odometer doesn't have the "up to 10% + 2.4 mph" variance - it's accurate.

#2 - I'm sure there would be a way to adjust the computer for tire sizes so that the computer and odometer remain accurate. I don't know if that would affect the % inaccuracy of the speedometer; depends on whether the speedo gets its signal from the computer, from the same source as the computer, or from somewhere else.
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      10-08-2009, 10:44 AM   #17
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Quote:
Originally Posted by willysp View Post
What does #1 mean? If you're refering to odometer miles, please see the posts above. The odometer doesn't have the "up to 10% + 2.4 mph" variance - it's accurate.

#2 - I'm sure there would be a way to adjust the computer for tire sizes so that the computer and odometer remain accurate. I don't know if that would affect the % inaccuracy of the speedometer; depends on whether the speedo gets its signal from the computer, from the same source as the computer, or from somewhere else.
I've been trying to be less long winded, but apparently I was too brief. LOL

1. In the old days, there was a physical cable that turned a physical set of gears that reflected a display of miles on it - the odometer. The analog odometer and the speedometer were connected. If one was wrong so was the other. So, I understand now there is no physical cable. It's sensors transmitting data to a computer (digital) that translates that data to an analog device - the speedometer. Last I watched a ‘tube vid of someone bumping up against BMW’s max speed limiter the speedometer read 150 mph, not 138 mph.

So, did the computer actually limit the car’s speed at a true physical speed of 138 mph and reflect 150 mph on the speedometer, because BMW states their limiter kicks in at 150 and so the speedometer should have read approximately 162 mph. Haven't heard anyone bragging their car can actually exceed the limiter. My point is, sure it possible for them to intentionally send a higher signal to the speedometer, but has anyone really checked that the odometer is correct?

That could have a detrimental effect on our warranties.

2.Given No. 1, above, I’m curious how do you go about verifying the right tire size code is in the computer.

Last edited by teagueAMX; 10-09-2009 at 02:35 PM..
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      10-09-2009, 12:17 PM   #18
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Road & Track test drove the 35i 0-60 mph in a published time of 4.8 seconds. Does this mean that the 3.5i goes only goes from 0-56 mph in 4.8 seconds, not to 60 mph, assuming they test drove one with an innacurate speedometer?
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      10-09-2009, 02:40 PM   #19
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Quote:
Originally Posted by BlueZ4AZ View Post
Road & Track test drove the 35i 0-60 mph in a published time of 4.8 seconds. Does this mean that the 3.5i goes only goes from 0-56 mph in 4.8 seconds, not to 60 mph, assuming they test drove one with an innacurate speedometer?
Good point and one would hope they used independent test gear.

Given my naturally suspicious nature, and constant quest for conspiracies , wouldn't it be interesting if someone had figured out a way to actually shorten the warranty mileage by 8 to 10 percent. Not claiming anyone actually did it,. . .FWIW
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      10-09-2009, 03:07 PM   #20
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Quote:
Originally Posted by BlueZ4AZ View Post
Road & Track test drove the 35i 0-60 mph in a published time of 4.8 seconds. Does this mean that the 3.5i goes only goes from 0-56 mph in 4.8 seconds, not to 60 mph, assuming they test drove one with an innacurate speedometer?
They use GPS:

The system that we use to record-braking data is the same we use for acceleration testing, our GPS-based VBOXIII by RaceLogic. This system samples our location 100 times a second in three dimensions using anything from six to 12 satellites for triangulation, and therefore can calculate distance, speed and acceleration in virtually every direction (though the X and Y coordinates are most accurate).

http://www.roadandtrack.com/article....rticle_id=8155
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      10-09-2009, 03:15 PM   #21
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Quote:
Originally Posted by teagueAMX View Post
Good point and one would hope they used independent test gear.

Given my naturally suspicious nature, and constant quest for conspiracies , wouldn't it be interesting if someone had figured out a way to actually shorten the warranty mileage by 8 to 10 percent. Not claiming anyone actually did it,. . .FWIW
The BMW odometers are accurate.

I have a trip that I make that is 105.9 miles in my old Z3, old Ford Explorer, current Nissan Armada, and new Z4. Each trip odometer read /reads within a couple tenths of a mile of 105.9 - i.e. less than .5% variance. Nissan says 105.9, Z4 says 105.7.
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      10-09-2009, 03:20 PM   #22
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Quote:
Originally Posted by willysp View Post
The BMW odometers are accurate.

I have a trip that I make that is 105.9 miles in my old Z3, old Ford Explorer, current Nissan Armada, and new Z4. Each trip odometer read /reads within a couple tenths of a mile of 105.9 - i.e. less than .5% variance. Nissan says 105.9, Z4 says 105.7.
Good news!
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