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Old 05-22-2019, 07:08 AM
Jim328 Jim328 is offline
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Actual speed vs. corrected speed

2008 328xi

Carly gave me an option to see "actual" speed in my little display, while the "analog" speedo shows "adjusted" speed.
"Actual" seems generally 2-3mph lower than "adjusted".

Adjusted for what?
Which one is real?
Can I display the speed the police think I'm going?

I'm embarking on a road trip (Denver to Vegas) tomorrow and I'll get a chance to compare both to my GPS.

I don't know how accurate the GPS speed display is, either.

How fast am I REALLY going?
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Old 05-22-2019, 07:52 AM
08 335Ci 08 335Ci is offline
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BMW spedo's (as most cars) are programmed 3 to 5 mph faster than actual mph. Helps to reduce your speed fines they feel. So the 'actual' mph doesn't have this built in speed warning.

Easiest way to check your spedo and GPS is to get on the interstate, set cruise control for 60 mph and time yourself over 1 mile. (S=D/T) The GPS in my Escort is pretty damn accurate.
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Old 05-22-2019, 08:34 AM
Jim328 Jim328 is offline
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So when the cop says, "Do you know how fast you were going?", I can honestly say, "I have absolutely no idea."

I'll do some measuring on my drive.
70W thru the mountains isn't really conducive to constant-speed runs, but things do flatten out a bit once I get closer to Vegas.
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Old 05-22-2019, 09:34 AM
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Originally Posted by Jim328 View Post
So when the cop says, "Do you know how fast you were going?", I can honestly say, "I have only a general idea."
Fixed. If the speedo indicates 80mph you're sure as s**t not ticking along at a serene 65.

The topic of BMW speedometer accuracy is even older than Internet hand-wringing over same. Search this site for tons of discussion, including references to the BMW SIB (service bulletin) that explains the maximum error allowed by ECE regulation (up to +4kph plus up to +10% of true speed).
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Old 05-22-2019, 10:00 AM
Jim328 Jim328 is offline
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I'm not THAT worried about it; this is the first vehicle I've owned that gave me an option on adjusted vs. actual.
It's not like I can outrun the Colorado Highway Patrol anyway; they have helicopters.
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Old 05-22-2019, 11:13 AM
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It's not like I can outrun the Colorado Highway Patrol anyway; they have helicopters.
...and radios. If you can outrun those, speedometer accuracy is moot.
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Old 05-22-2019, 12:54 PM
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Yup. On a clear day and straight road, my e92 might just be able to outrun a chopper (see the movie Cannonball Run LOL). Except for them pesky radios, of course.
As said, there are many earlier threads about how the spedos are set to indicate faster. Short Version: European laws put the liability for speedo errors on the manufacturer. So if it indicates 110 kph but you're actually driving 120 because it's indicating 10% slow, then the car's manufacturer can be sued. So they set them to indicate about 5-8% slow to give themselves a safety margin to allow for calibration drift. I understand this is very common for European cars.
Also, the GPS speed displayed pretty much has to be very accurate, as it compares location to known maps. If it was wrong, you'd drive off that cliff.
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Old 05-22-2019, 01:20 PM
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Originally Posted by Zeichen311 View Post
...and radios. If you can outrun those, speedometer accuracy is moot.
"Prepare ship for light speed!"
"No, no, no, light speed is too slow!"
"Light speed, too slow?!"
"Yes, we're gonna have to go right to….ludicrous speed!"
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Old 05-22-2019, 02:31 PM
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Real answer. If you use Carly to display the actual speed, it will display down in the area where it also displays your average speed, average mpg, etc. The way you get it to display is to cycle through those displays and one of the last ones to show up is the MPH readout. It will look something like this V= 67 I assume V means velocity. When I set my cruise control at 73 the real velocity readout is 70.
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Old 05-22-2019, 03:21 PM
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Originally Posted by rdkind62 View Post
Real answer. If you use Carly to display the actual speed, it will display down in the area where it also displays your average speed, average mpg, etc. The way you get it to display is to cycle through those displays and one of the last ones to show up is the MPH readout. It will look something like this V= 67 I assume V means velocity. When I set my cruise control at 73 the real velocity readout is 70.
If you use INPA to code the KOMBI module, the digital speed read out has to be turned on. then there's 2 actual speeds to enable. One for the digital speed readout and one for the analog speedometer. There's also an adjustment for different tire sizes for both the spedo and odometer.

I was able to get my spedo both digital and analog to within 1 mph of my Escort's GPS readout.
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Old 05-23-2019, 08:51 PM
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The BMW E90 analog speedometer runs about 4% slow (optimistic) by design, and there is NO CORRECTION possible for this error.

IF you enable the DIGITAL speedometer display, the 'adjusted' speed mimics the optimistic analog gauge, but when you use Carly to change that setting, the digital speedometer is close to right on. But a change of tires CAN increase/decrease the percentage of error based upon the rolling diameter of the tire. My digital speedometer was in agreement with GPS, but once I switched to Bridgestone DriveGuard tires the indicated digital speed indicates 1mph optimistic at about 75mph.
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Old 05-24-2019, 06:20 AM
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Originally Posted by wilt View Post
The BMW E90 analog speedometer runs about 4% slow (optimistic) by design, and there is NO CORRECTION possible for this error.

IF you enable the DIGITAL speedometer display, the 'adjusted' speed mimics the optimistic analog gauge, but when you use Carly to change that setting, the digital speedometer is close to right on. But a change of tires CAN increase/decrease the percentage of error based upon the rolling diameter of the tire. My digital speedometer was in agreement with GPS, but once I switched to Bridgestone DriveGuard tires the indicated digital speed indicates 1mph optimistic at about 75mph.
The 'analog' speedometer on our bimmers is not like the old mechanical cable and magnetic clutch speedos of years past, but are a analog display of digital information. And being such ARE adjustable through coding of the KOMBI module. See attached jpg of the KOMBI module as read in NcsDummy. The DME counts the impulses supplied to it, then tells the speedo what to display. Change the count, the speedo changes displayed speed.

I've never seen anyone complain about a BMW speedo 'running slow'. They usually complain they display a FASTER than actual speed. 4% of 60 mph is 2.4 mph. which is below the 5 mph error most owners complain about.

A change in tire diameter through either a combination of wheel or aspect ratio WILL change the speedo reading. A taller tire (larger dia.) will make the speedometer read slower than previous and a smaller tire the speedo reads faster.
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No matter how fast I go, there's always someone slower in front of me.

2010 E92 335 M Sport, Titan Silver Metallic, Steptronic, JB-4 G5 ISO, WMI, 3.5 Tmap (22 psi boost), BEF, Front Strut Brace, SS brake lines, Drilled and Slotted Rotors, Anti-lag, OCC, Escort Max 360

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Old 05-24-2019, 06:01 PM
wilt wilt is offline
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I wrongly described the analog display as 'slow'...I meant that an indicated 70mph was actually 67mph over the ground (therefore, 'optimistic'); if you thought you were going 65mph per the signage, the driver behind you thought you were driving slow!
I have found that at 90mph indicated (during long runs in NV on way to UT), the speed over the ground was 86mph, so not a simple 3mph 'offset' from real speed but 4mph optimistic at that speed. And in switching the Bridgestone DriveGuards in the past year, even the digital readout is now about 1mph optimistic (worse than with factory Continentals)

Last edited by wilt; 05-24-2019 at 06:05 PM.
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Old 05-24-2019, 08:46 PM
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I have found that at 90mph indicated ..., the speed over the ground was 86mph, so not a simple 3mph 'offset' from real speed but 4mph optimistic at that speed.
If it's been said once it's been said a hundred times here on the 'Fest alone: The speedometer error is not a fixed offset.

The potential error has both fixed and proportional components. The indicated speed can be in error by a constant 0 to +4kph (+2.4mph) plus 0 to +10% of true speed. The amount of the latter component increases as true speed increases.
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Old 05-28-2019, 08:08 AM
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The speedometer error is not a fixed offset.
I have discovered (with the aid of the Utah State Patrol) that when the analog speedo shows 91, I'm actually going 88, which matched the digital display.
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Old 05-28-2019, 08:26 AM
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Old 05-28-2019, 08:26 AM
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Scepticism is the chastity of the intellect, and it is shameful to surrender it too soon or to the first comer: there is nobility in preserving it coolly and proudly through long youth, until at last, in the ripeness of instinct and discretion, it can be safely exchanged for fidelity and happiness.
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Old 05-28-2019, 09:18 AM
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Three and you go free. Nine and you’re mine. Smokey da bear.
LOL.
Colorado Highway Patrol's ticket books used to start at 5 over the limit.
That was years ago, don't know what the deal might be now.
I generally set cruise at 5 over.
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Old 05-28-2019, 09:26 AM
Robert A Robert A is offline
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Keep in mind that your car's speedometer does not know the size of your tires, so even "actual" is subject to error.

I find it odd that BMW can't produce a 100% accurate speedo, at least on newer gen cars with built in GPS.

Quote:
Originally Posted by Jim328 View Post
2008 328xi

Carly gave me an option to see "actual" speed in my little display, while the "analog" speedo shows "adjusted" speed.
"Actual" seems generally 2-3mph lower than "adjusted".

Adjusted for what?
Which one is real?
Can I display the speed the police think I'm going?

I'm embarking on a road trip (Denver to Vegas) tomorrow and I'll get a chance to compare both to my GPS.

I don't know how accurate the GPS speed display is, either.

How fast am I REALLY going?
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Old 05-28-2019, 09:55 AM
Jim328 Jim328 is offline
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I will further point out that my car feels exactly the same at 100mph as it does at 40mph.
So it's hardly my fault I was going 100 in a 40 zone.
hahhahahahha
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Old 05-28-2019, 09:55 AM
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I will further point out that my car feels exactly the same at 100mph as it does at 40mph.
So it's hardly my fault I was going 100 in a 40 zone.
hahhahahahha
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Old 05-28-2019, 12:02 PM
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I find it odd that BMW can't produce a 100% accurate speedo, at least on newer gen cars with built in GPS.
GPS cannot be used for the primary vehicle speedometer because a position fix is not always available. When there is no signal or too few satellites are in view, the car needs to rely on mechanical (or at least, fully on-board) speed measurement and accelerometer data for dead-reckoning navigation.
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Old 05-28-2019, 12:08 PM
Robert A Robert A is offline
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I agree, but GPS can be used to reliably calibrate a wheel-based measuring device. That's the way it works on my cycling computer, and it's accurate to within 1/10th of a percent

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Originally Posted by Zeichen311 View Post
GPS cannot be used for the primary vehicle speedometer because a position fix is not always available. When there is no signal or too few satellites are in view, the car needs to rely on mechanical (or at least, fully on-board) speed measurement and accelerometer data for dead-reckoning navigation.
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Old 05-28-2019, 12:27 PM
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I agree, but GPS can be used to reliably calibrate a wheel-based measuring device. That's the way it works on my cycling computer, and it's accurate to within 1/10th of a percent
The other factor--in truth, the more important one--is that not all cars are equipped with GPS navigation equipment.
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Old 05-28-2019, 12:45 PM
Robert A Robert A is offline
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True, but I'm pretty certain that every car has BMW Assist with GPS tracking.
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Originally Posted by Zeichen311 View Post
The other factor--in truth, the more important one--is that not all cars are equipped with GPS navigation equipment.
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