BMW 1 Series Coupe Forum / 1 Series Convertible Forum (1M / tii / 135i / 128i / Coupe / Cabrio / Hatchback) (BMW E82 E88 128i 130i 135i)
 





 

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      06-18-2009, 02:14 AM   #1
qship
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How accurate is your speedo?

I;m up bush at the moment and had a chance to compare my GPS with the speedo of the car.
96 on the GPS = 100 on speedo
114 on GPS = 120 on speedo

Anyone else tried this?

The reading seem about right given the fudge factor built in by manufactuers these days.
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      06-18-2009, 02:21 AM   #2
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Sounds about right with the 5% variance..
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      06-18-2009, 02:22 AM   #3
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Usually its the other way round.

Your speedo sits around about 5-7km below what your actual true speed reading is...
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      06-18-2009, 02:23 AM   #4
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Comparisons to my GPS seem to agree with you qship.
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      06-18-2009, 02:41 AM   #5
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Quote:
Originally Posted by OOOSQ View Post
Usually its the other way round.

Your speedo sits around about 5-7km below what your actual true speed reading is...
you sure about this? pretty sure most car speedos over estimate.
i assume the car companies would be in a lot of strife if they were to underestimate the true speed...
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      06-18-2009, 03:26 AM   #6
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Quote:
Originally Posted by jksgoh View Post
you sure about this? pretty sure most car speedos over estimate.
i assume the car companies would be in a lot of strife if they were to underestimate the true speed...
+1. I've yet to see a review which states that the speedo underestimates the true speed. I agree that the car companies would be in trouble and even a law suit. For example if you get booked for speeding but you were adamant that the speedo was reading the speed limit. It would be safer for driver and car company to over estimate the actual speed.
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      06-18-2009, 03:33 AM   #7
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ADR's now state that speedo readings have to be -0%/+10%, i.e. they can read fast, but not slow.
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      06-18-2009, 03:34 AM   #8
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Same here, about 5% out. So is my X5 and so was my M5.
It seems pretty std with BMW's.
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      06-18-2009, 03:36 AM   #9
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Quote:
Originally Posted by OOOSQ View Post
Usually its the other way round.

Your speedo sits around about 5-7km below what your actual true speed reading is...
Its the right way round...Your speedo should be over estimating your speed..
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      06-18-2009, 03:47 AM   #10
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I have almost the exact same results as you are showing.
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      06-18-2009, 03:54 AM   #11
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60 on speedo, 56 on police roadside radar billboard thingo

cops placed it just over a crest so it's hard to maintain speed, how stupid
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      06-18-2009, 04:36 AM   #12
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Quote:
Originally Posted by OOOSQ View Post
Usually its the other way round.

Your speedo sits around about 5-7km below what your actual true speed reading is...
he just had a long day
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      06-18-2009, 04:41 AM   #13
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Quote:
Originally Posted by sparoz View Post
he just had a long day

Maybe his car can't go past the speed limit?

Long time no see Sam!
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      06-18-2009, 07:13 AM   #14
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or, your gps is out
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      06-18-2009, 10:29 AM   #15
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My "legal" speedo cruise controls speeds are 108km/h in 100 zone and 119km/h in 110 zone. My GPS at each of those speeds reads 103km/h and 114km/h respectively, which are a few k's inside the Vic Police tolerance
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      06-18-2009, 04:40 PM   #16
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Several years ago, I approached my dealer asking why the speed limit on my E46 only 'gongs' beyond the speed I set. In other words, if I set it to 70, it chimes at about 74km/h.

I also noticed that if I press the 'set limit to current speed' (or words to that effect) that it would always set a speed slower than what the speedo was showing at the time, ie I need to be going about 74km/h for it to set to 70km/h.

I haven't looked but assume these features are similar on the E82.

At that time, Service Manager pulled up a bulletin from BMW explaining the built-in error factor which is deliberate and 'normal'. It would appear the above 'limit' test shows up this built-in error factor as equipment like factory GPS relies on 'accurate' input. So what we see there is a way to show up 'real' speed versus speedo 'reported' speed.

In summary, the car knows exactly how quickly it is going, but the speedo is designed to be optimistic. What you can do to test this theory is to cruise at the same speedo speed as when you performed the tests below, and then to set the limit speed to your speed. I think you will find the speed it sets will correspond quite closely to your portable GPS and/or street speed test signs.

By the way, in case you are wondering 'Cruise Control' sets itself to speedo speed, not actual speed.
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      06-18-2009, 06:12 PM   #17
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Some more data for you HERE qship. I spent a day playing with my Performance Box checking speeds in various gears. Cruise speeds of 60, 70, 80km/hr on the GPS box (set with cruise control) corresponded to 63, 73, 84km/hr on the speedo.
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      06-18-2009, 06:39 PM   #18
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I always thought GPS/NAV equipped cars should have an 'auto calibrate' feature built into the speedo, to make them 100% accurate. Wouldn't be that hard to do considering it's just a stepper motor in there nowadays.
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      06-18-2009, 10:13 PM   #19
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Quote:
Originally Posted by MickyD View Post
I always thought GPS/NAV equipped cars should have an 'auto calibrate' feature built into the speedo, to make them 100% accurate. Wouldn't be that hard to do considering it's just a stepper motor in there nowadays.
I agree. Or they could install Pitot tubes in the front
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      06-18-2009, 10:20 PM   #20
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Quote:
Originally Posted by d3syd View Post
I agree. Or they could install Pitot tubes in the front
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      06-19-2009, 01:32 AM   #21
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Mine is out by 6-7kmph, according to a couple of freeway speed check displays. Seems like this is the norm.
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      06-21-2009, 03:15 AM   #22
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Mine is 4km/h over the GPS speed. Seems to be this at all speeds.
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