Built in Speedo error

S

Stuart Truman

Lifer
 The Cotswolds
I notice BMW have built a hell of a safety margin over-read on the speedo compared with a gps track in a straight line.

On a VW you can turn this off via coding. Anyone know if it's possible to turn this off in a BMW? I'd rather know what speed I'm really doing. On mine, 83 on the speedo is 78 for real. Tested on a track of course *innocent look*
 
The bigger rear I put on my ///M (265/40r18 rather than 255/40r18) has balanced it out a bit, and is about 3% out at 100mph now.
 
mmm-five said:
The bigger rear I put on my ///M (265/40r18 rather than 255/40r18) has balanced it out a bit, and is about 3% out at 100mph now.
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X2 and exactly what I have done...
 
Stuart Truman said:
I notice BMW have built a hell of a safety margin over-read on the speedo compared with a gps track in a straight line.

On a VW you can turn this off via coding. Anyone know if it's possible to turn this off in a BMW? I'd rather know what speed I'm really doing. On mine, 83 on the speedo is 78 for real. Tested on a track of course *innocent look*
Click to expand...

Just how did you calculate this - with a Tom Tom or similar? I'd be very keen to find out the exact reading on mine before I get flashed by a camera or similar...
 
Easy enough with any GPS system. I use a simple app on an iphone that give constant MPH readout in MPH. Def best tested on straight flat motorways to limit any error.

Mine's out the same sort of amount as posted above.
 
Stuart Truman said:
I notice BMW have built a hell of a safety margin over-read on the speedo compared with a gps track in a straight line.

On a VW you can turn this off via coding. Anyone know if it's possible to turn this off in a BMW? I'd rather know what speed I'm really doing. On mine, 83 on the speedo is 78 for real. Tested on a track of course *innocent look*
Click to expand...


Compared to some other manufacturers that's not too bad! I remember Vauxhall in particular used to be way out. Autocar used to have the speedo reading versus true mph in 10 mph increments when they road-tested cars and the Vauxhalls used to be showing 99 mph at a true 90mph.

I think mine is pretty accurate up to 70 mph. IIRC at a true 30mph I was showing 31mph on the speedo last time I checked.
 
TINO said:
mines about 2mph over what sat nav says. :D
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Mine too. My E46 was 4mph out. My daily runner (Polo) is 7/8mph out when doing 70mph!! :o


Sent from my iPhone using Tapatalk
 
Most cars are around 4mph out. Think of it this way, it comes in handy if you go past a speed camera not paying attention slightly over the limit, because you'll probably be closer (or even under) the limit than you think!
 
This could be worth a check, having changed the wheel/tyre size on both cars... better to be safe than sorry!
 
Stuart - you can set the intravee to show the speed on your satnav screen (with the extras menu). Mine seems to match the gps reading, as well as the speed reader in the next village. I use that as a measure of actual speed, whereas the speedo has the same saftey margin built in that yours is showing
 
mmm-five said:
The bigger rear I put on my ///M (265/40r18 rather than 255/40r18) has balanced it out a bit, and is about 3% out at 100mph now.
Click to expand...

I'm confused. 265 vs 255 is wider, not a bigger diameter so how does this make a difference? You're still running an 18" tyre with a 40mm profile so my simple maths says nothing has changed to effect the speedo reading. The only thing that should do this would be the radius of the centre of the wheel to the road surface, so in theory a new tyre would reduce the error until its tread wears down again.
 
markeg said:
Stuart - you can set the intravee to show the speed on your satnav screen (with the extras menu). Mine seems to match the gps reading, as well as the speed reader in the next village. I use that as a measure of actual speed, whereas the speedo has the same saftey margin built in that yours is showing
Click to expand...

This sounds promising (if my stuff ever arrives *sigh*) I wonder if the Intravee is taking a reading from the GPS or from the real speed sensor before the computers mess it up? If that's the case I guess it can be coded out.
 
Stuart Truman said:
I'm confused. 265 vs 255 is wider, not a bigger diameter so how does this make a difference? You're still running an 18" tyre with a 40mm profile so my simple maths says nothing has changed to effect the speedo reading.
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The 40 is profile height as a percentage of width, not a fixed dimension. So a tyre 10mm wider will be 40% * 10mm = 4mm taller in profile, and thus 8mm taller in overall height.
 
It's quite interesting the effect that wheel size has on speedo accuracy, when I changed from 17" to 18" my speedo became more accurate, so given the same width, diameter increases as the wheels become bigger/smaller profile and so the circumference increases and therefore the distance covered increases per one revolution of the wheel

Hence all manufacturers have to account for speedo error due to change in thread depth (10 mm or so) and change in wheel size in the options list.
 
GaryH said:
Most cars are around 4mph out. Think of it this way, it comes in handy if you go past a speed camera not paying attention slightly over the limit, because you'll probably be closer (or even under) the limit than you think!
Click to expand...
It doesn't quite work that way, as the whole range is a certain percentage out, about 5% in my measurements (based on setting the cruise and letting the sat-nav settle for 10 seconds).

Before the tyre change:
  • 30 on the clock = 31 on the satnav
    42 = 40
    53 = 50
    63 = 60
    85 = 80
    106 = 100
    127 = 120
    148 = 140
    170mph = 160mph

With the tyre change, mine now reads:
  • 30 = 30
    41 = 40
    62 = 60
    83 = 80
    103 = 100
    124 = 120
    144 = 140
    165mph = 160mph
 
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