SAt nav vs speedo for "real" speed

[mcbeee]

I'm curious, how many people here use a GPS or onboard Nav system to tell true speed vs the one shown on the speedo ???

My M was 6 clicks over the true speed, my Z3 was 7 and my new one is 6 over as well. None had the normal wheels, all factory upgrades though. My Gf's 325 is dead on the money with the original wheels (not upgraded). You'd think with all the hi-tech stuff in these cars BMW could figure out a simple offset in the electronics to adjust this. or, maybe they have and nobody told me....

 

[Redzedfour]

I think speedo's are calibrated to show a small percentage under the actual speed,regarding the satnav that is the true speed and i always go by that for example average speed cameras go by the nav and you can overtake other cars who go by the speedo reading,have been going by the nav for years and not had a problem hope that was of some help

 

[cj10jeeper]

I think speedo's are calibrated to show a small percentage under the actual speed,regarding the satnav that is the true speed and i always go by that for example average speed cameras go by the nav and you can overtake other cars who go by the speedo reading,have been going by the nav for years and not had a problem hope that was of some help

Did you get that the right way around? If anything speedos are set to show a slightly higher speed than actual. Flatter the driver and car performance, avoids manufacturer liability, build in a margin for error, etc.

GPS speed is really accurate if it has long enough to work and there are no corners or significant inclines in it, else it's way out.

 

[Redzedfour]

Did you get that the right way around? If anything speedos are set to show a slightly higher speed than actual. Flatter the driver and car performance, avoids manufacturer liability, build in a margin for error, etc.

GPS speed is really accurate if it has long enough to work and there are no corners or significant inclines in it, else it's way out.[/quote]


I think i got that the right way round.For example setting the speed to 50mph (using cruise) as indicated on the speedo and then checking the speed on the satnav and it indicates 2-3 mph less.There is not much point calibrateing the speedo to be over as you could be nabbed for speeding as your actual speed would be over the designated speed limit,2-3mph over the speed limit thats enough to get you a ticket as i know from experience(wasn't using the satnav at that particular time :thumbsdown: )

 

[mcbeee]

To add a little more info: I drive a fair bit in the USA with a Canadian BMW, my speedos are all KMH and the USA is still in MPH. So subtracting a 6 click offset then converting KPH to MPH in your head while zipping down the road at 75 or 80MPH, plus the offset is dependent on the speed, it gets very f*in tedious. That's why I have been using my GPS systems for years and wonder why BMW can't deal with this. It wouldn't be that difficult !!

 

[vachss]

I did measure with a GPS once on my 3.0si coupe. I got a consistent 4% difference with the speedometer reading 4% over from 50-100 mph (i.e. 80 indicated is really 77, 100 indicated really 96, etc.)

 

[jamiez]

Yep - kinda same here in an e36 i own with a mate - 328i sport
We did a trip to germany recently and we went flat out - speedo read 151 yet GPR reported 147 - which is exactaly what the manual says - not bad for a 10 year old car with zero mods!

 

[lacroupade]

Did you get that the right way around? If anything speedos are set to show a slightly higher speed than actual. Flatter the driver and car performance, avoids manufacturer liability, build in a margin for error, etc.

GPS speed is really accurate if it has long enough to work and there are no corners or significant inclines in it, else it's way out.

I think i got that the right way round.For example setting the speed to 50mph (using cruise) as indicated on the speedo and then checking the speed on the satnav and it indicates 2-3 mph less.There is not much point calibrateing the speedo to be over as you could be nabbed for speeding as your actual speed would be over the designated speed limit,2-3mph over the speed limit thats enough to get you a ticket as i know from experience(wasn't using the satnav at that particular time :thumbsdown: )

Thats a bit of female logic there LOL....I think you are violent agreeing if you think about it. Speedo always OVERREADS yes? which is what redzed is saying....

And @CJ, Ive been pondering your assertion and I'm going to argue with it cos I don't think thats how GPS works, although units vary in their operation. I'm assuming your logic here is that, for example, an overhead view of a one mile stretch of straight road and a one mile stretch of hill at 45 degrees is very different? But satnav does two things to counteract that which I am confident give a wholly accurate reading - because remember GP is a military system, not civilian, so was designed to provide accurate positional and movement data. Firstly, because is normally working off multiple satellites its three-dimensional, and secondly, its sampling your position much more often than you think.

On a Garmin eTrex, for example, you can set the unit at whatever resolution you like - sampling position every 5-10 yards if you wish. At the other extreme you can do it every mile, in which case I agree, because you could have done that mile from A to B via Lands End and the unit wouldn't know, so would miscalculate your speed.

I suspect, given the reliance on them, that TomTom and the like are sampling somewhere between extremes like that....given the refresh rate 'Ive seen I suspect it to be of the order of 50-100yds but who knows?? In fact having just written that, I've googled and I'm probably not far off - PocketGPS says the TT refreshes every second apparently, so can be up to 0.5% inaccurate but such a rate, and the 3-D element pretty much takes care of all things, meaning your in-car GPS is fairly accurate regardless of terrain.

 

[cj10jeeper]

Yep - we are agreeing in subsequent explanations, but just phrasing things differently. When RedZedfour opened with "I think speedo's are calibrated to show a small percentage under the actual speed" then that was plain wrong. :thumbsdown:

You could well be right on the GPS and refresh rates and ability to adjust to terrain. I'm not convinced about it's accuracy through say a set of S bends and in a previous thread nobody managed to post if it calculates altitude or estimates it based upon a known position and database of height at that point (poor explanation). The BMW system for instance acknowledges the frequent loss of signal and to continues to give directions, in such instances, uses inputs from the ABS to calculate relative speed of rotation of the rear wheels and from that estimate distance and direction.

I certainly note that when ever I use one of the hand held it takes several seconds to adjust correctly to any new speed set in the car.
Of course once set after a few seconds I assume it's pinpoint accurate if holding a good few sattelites and therefore on say a motorway is perfect to check speed.

I think what we can be certain of is that GPS is a great tool to allow you to get a feel for what you real speed is and drive to the permitted limit, such as average speed controlled zones

 

[mmm-five]

I know my TomTom can't always keep up with me as sometimes I'll be past a junction on a roundabout before it's had chance to update either the display or announce the turn

It's also slow to register speed changes, such as when you pull away from lights, or pull out to overtake. It seems to lag about a second or so behind, and that's why I always give it a couple of seconds to adjust before setting my CC when I'm in a camera-controlled speed limit.

I did a test with mine earlier in the year...
http://www.z4-forum.com/forum/viewtopic.php?p=139976#p139976

 

[lacroupade]

Yep - we are agreeing in subsequent explanations, but just phrasing things differently. When RedZedfour opened with "I think speedo's are calibrated to show a small percentage under the actual speed" then that was plain wrong. :thumbsdown:

You could well be right on the GPS and refresh rates and ability to adjust to terrain. I'm not convinced about it's accuracy through say a set of S bends and in a previous thread nobody managed to post if it calculates altitude or estimates it based upon a known position and database of height at that point (poor explanation). The BMW system for instance acknowledges the frequent loss of signal and to continues to give directions, in such instances, uses inputs from the ABS to calculate relative speed of rotation of the rear wheels and from that estimate distance and direction.

I certainly note that when ever I use one of the hand held it takes several seconds to adjust correctly to any new speed set in the car.
Of course once set after a few seconds I assume it's pinpoint accurate if holding a good few sattelites and therefore on say a motorway is perfect to check speed.

I think what we can be certain of is that GPS is a great tool to allow you to get a feel for what you real speed is and drive to the permitted limit, such as average speed controlled zones

I suspect your right about general accuracy - the eTrex is a much more specialist walkers tool that needs that degree of accuracy. In-car satnav is more idiot proof I guess.

But for the avoidance of doubt, unequivocally, whetever system it is, GPS is mapping your position to within 10-20 feet in all three dimensions, so knows your exact height and relative position in space as it were at all times, but again, its a question of whether basic in-car units take advantage of that data isn't it. One has to assume they must as the degree of error is otherwise potentially quite high....especially if you live somewhere like Switzerland!

Given the already remarked-upon issue that too many of these units like to just lose their signal for no apparent reason (satellite comms can be flaky at the best of times even with bleeding-edge military hardware), your original comment about time being needed to acquire sensible data is pertinent; its worth being aware.

I've got to say, I think that might be a bit of an urban myth about the BMW satnav 'carrying on regardless' though (happy to be corrected if someone knows the factory answer?) - TomTom does the same for a while as well, but I think it just assumes you are continuing at the last known rate of progress, because having been caught in a Paris subway jam several times I've watched Mr Tom carry on down the road until he reaches that "oh sh*t" point and finally admits to losing the signal a minute or two later....!!

In fact if anyone with a factory satnav has ever had that "TomTom" moment where your icon suddenly appears to be in the middle of a ploughed field, I guess that would prove my theory....

 

[a11y]

To add a little more info: I drive a fair bit in the USA with a Canadian BMW, my speedos are all KMH and the USA is still in MPH.

I wonder why BMW don't use a similar idea to what they do with our (UK) speedo: put both MPH and KPH on the speedo display?

My Z4 has the most accurate speedo of any car I've owned - by that I mean it matches my TomTom GPS unit speed closely: the Z4 displays 2-3mph less than the GPS speed at 70mph.

My old Clio was bizarre. The speedo read 4-5mpg less than GPS speed at 70mph, which is fair enough. But if you set the cruise control to 70mph, it'd display your cruise speed as 70mph in the display but your speedo needle would sit around 74-75mph! It made me wonder if there was some GPS system for the cruise control, although it was probably just dodgy French electrics :roll:

 

[Andy]

if you go into the hidden display under the speedo, you can get an actual speed reading - assuming the car has the correct wheels / tyres etc, the car actually knows the speedo is over reading, iirc. I've never tried it and because of the keypresses I doubt you'd do this when you were moving.

as Phil says, the oem nav definitely uses abs sensors to supplment gps, as they can need re-calibrating if you do things like donuts which totally confuses it, meaning the abs and gps contradict each other. there's a factory reset documented somewhere which basically says drive around town in the open, taking lots of lefts and rights at junctions and they will re-syncronise again.

 

[a11y]

if you go into the hidden display under the speedo, you can get an actual speed reading - assuming the car has the correct wheels / tyres etc, the car actually knows the speedo is over reading, iirc. I've never tried it and because of the keypresses I doubt you'd do this when you were moving.

That could be interesting. Is that an M-model specific thing? I really can't recall seeing it in my 3.0i SE...

 

[Andy]

it's hidden ally

I never tried it but I'm pretty sure it works. I won't post it here coz it's too big, but here's the link...
http://www.z4-forum.com/forum/viewtopic.php?f=34&t=15176&p=222820#p222820

 

[cj10jeeper]

Lacroupade - sorry - not an urban myth - here is a small absract from the factory documentation on the Sat Nav system:

Note the references to rear wheel ABS to input speed and direction of rotation and use of a yaw rate sensor for calculating position while driving. It actually implies in this that for the built in sat nav the GPS is secondary to the wheel and yaw sensors, however I know from other documentation that the GPS checks against the calculated position and corrects as errors develop. (Bet chaged wheel rotational size plays havoc if not recoded)

I can email you the large pdf if you fancy a read oke:

Navigation computer DVD
The navigation computer is located in the lockable compartment between the seats in the
partition between the vehicle interior and luggage compartment.
The navigation computer DVD incorporates the main functions for the navigation system.
and contains the following components:
• DVD drive for reading information of the digitized road maps on DVD or CD
• GPS receiver for determining position before start of journey
• Yaw rate sensor for calculating position while driving
• Powerful processor for fast calculation of data
• Memory modules for buffering data
• Interfaces for communication with other systems and evaluating sensor data
• Interfaces for outputting image data and voice information
• GPS aerial
GPS Aerial
The GPS aerial receives the signals from GPS satellites and transfers the data (degrees longitude,
degrees latitude, Greenwich meantime (GMT)) to the GPS receiver in the navigation
computer DVD for the purpose of calculating the position. This is of particular importance
during initial or re-initialization of the system. These data are also read in during the restart
procedure.
The GPS aerial is located in the middle of the roof frame behind the interior lamp.
Wheel speed sensor
The wheel speed sensor determines the rotary motion of the wheel. The information is routed
to the ABS/DSC control unit and the corresponding distance calculated. The conditioned
signal is then made available to the navigation computer. The ABS sensor at the rear
left is used for the purpose of determining the speed signal.

 

[mcbeee]

Quote: a11y "I wonder why BMW don't use a similar idea to what they do with our (UK) speedo: put both MPH and KPH on the speedo display?"

BMW USA do have both markings on their speedos, the km marks are wide scale and small. Almost useless, and also out of sync by 5 or 6 % I assume. Another little tidbit of info, if I import an American BMW into Canada, BMW Canada insists that the speedo is changed to the kmh only and charge $6000. Yes , six thousand $ to bring the car to BMW's Canadian standards. I believe that BMW is the only car company in Canada that is this crazy, the others can usually be modded for under a $1000, (usually 300 to 500). As the cars sell for about 10 grand less in the US this is price protectionism at its best !!

 

[RichardG]

I've noticed that my indicated BMW speed and Garmin speed are much closer than they used to be since changing from 17's to 19's,

if you go into the hidden display under the speedo, you can get an actual speed reading - assuming the car has the correct wheels / tyres etc, the car actually knows the speedo is over reading, iirc. I've never tried it and because of the keypresses I doubt you'd do this when you were moving.

However

I just wonder how much effort do BMW go to to calibrate each individual car's speedo to it's wheel size, bearing in mind OEM on a 3.0 is a 17" wheel (certainly pre-facelift), when looking at a link in this earlier post about the circumference of a tyre on different diameter wheels once the aspect ratio is taken into account it shows very little difference in size, similar almost to the same kind of difference as found between new and worn tyres if you take a new tyre as having say 7mm of tread, at its legal limit of 2mm its diameter has reduced by 10mm, which is bigger than difference inferred by the table between a 17" wheel fitted with a 225/45 tyre and a 265/30 tyre fitted to a 19" wheel.

Hope that makes sense

 

[lacroupade]

OK CJ - you win this time! LOL :kissass:

 

[cj10jeeper]

OK CJ - you win this time! LOL :kissass:

Better believe it mon ami