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  #121  
Old 12-29-2018, 12:36 AM
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Originally Posted by wpoll View Post
I took note of the speedo vs BC menu #8 V: value today...



I should have verified these values via a GPS at the same time, as these values are with a set of tyres at pretty much minimum tread depth. I want to perform the same check once I replace the tyres so I can gauge the variation over the life of the tyres. This will affect any correction factor I might code, as no one correction factor will read TRUE speed for the entire life of a set of tyres. I'd like to set a correction factor that doesn't ever result in a speedo reading UNDER the true speed...
What I noticed between used (very) tires and new and the gps speed vs the displayed speed was no difference. At 80 mph (gps), the speedo consistently reads 82/83 mph.
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  #122  
Old 12-29-2018, 12:42 AM
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Originally Posted by Fifty150hs View Post
What I noticed between used (very) tires and new and the gps speed vs the displayed speed was no difference. At 80 mph (gps), the speedo consistently reads 82/83 mph.
Interesting - I would expect to see a 3-4% variation over the life of the tyre. That's what I see on my bike and the X5 calculates speed in the same way - via the ABS sensors...

I guess it's possible there is a correction factor built in to allow for the tyre wear but that would require the original tyre size to be known... too complex, even for BMW.
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  #123  
Old 12-29-2018, 12:55 AM
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Originally Posted by wpoll View Post
Interesting - I would expect to see a 3-4% variation over the life of the tyre. That's what I see on my bike and the X5 calculates speed in the same way - via the ABS sensors...

I guess it's possible there is a correction factor built in to allow for the tyre wear but that would require the original tyre size to be known... too complex, even for BMW.
Tire diameter is set. It's virtually the same no matter what diameter wheel as long as the correct tire size is used with the expected diameter wheel. 17" wheel, taller tire. 20" wheel much shorter tire. But the over all diameter remains the same. That's how the tire pressure system works and it wouldn't work if there was any difference in overall tire diameter no matter what size wheel. It's the same for the speedo. If you throw a radically different overall tire/wheel diameter at it, it will not read remotely correctly and you will likely get a tire warning light. And your speedo will be off as well.
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  #124  
Old 12-29-2018, 12:59 AM
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Originally Posted by Fifty150hs View Post
Tire diameter is set. It's virtually the same no matter what diameter wheel as long as the correct tire size is used with the expected diameter wheel. 17" wheel, taller tire. 20" wheel much shorter tire. But the over all diameter remains the same. That's how the tire pressure system works and it wouldn't work if there was any difference in overall tire diameter no matter what size wheel. It's the same for the speedo. If you throw a radically different overall tire/wheel diameter at it, it will not read remotely correctly and you will likely get a tire warning light. And your speedo will be off as well.
Exactly. But the tyre diameter changes as it wears... not by much but it's enough...
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  #125  
Old 12-29-2018, 01:31 AM
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Quote:
Originally Posted by wpoll View Post
I took note of the speedo vs BC menu #8 V: value today...



I should have verified these values via a GPS at the same time, as these values are with a set of tyres at pretty much minimum tread depth. I want to perform the same check once I replace the tyres so I can gauge the variation over the life of the tyres. This will affect any correction factor I might code, as no one correction factor will read TRUE speed for the entire life of a set of tyres. I'd like to set a correction factor that doesn't ever result in a speedo reading UNDER the true speed...
If the true speed is calculated using new factory size tyres on the factory wheels, then , as the tread wears, it will only mean your speedo indicates higher as the diameter gets smaller with wear,
never the other way round,
unless of course you put larger diameter tyres on than standard.
Be interesting to how your indicated speed compares to GPS speed,
assuming thatís accurate.
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  #126  
Old 12-29-2018, 02:06 AM
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Originally Posted by wpoll View Post
Exactly. But the tyre diameter changes as it wears... not by much but it's enough...
In my experience it doesn't matter. Worn tire vs new it is virtually the same reading on the speedo.
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  #127  
Old 12-29-2018, 02:07 AM
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Nailed it, deepblonde. Which is why I won't calibrate the speedo to be precise (with respect to a GPS i.e. true speed) on worn tyres - that could result in a Speedo reason low when new tyres are fitted.

Of course, this isn't what we are talking about here - we are only talking about making the speedo dial match the calculated velocity value - and it's that calculated velocity value that I'm not sure about.

And I don't think that the calculated velocity value is adjustable - or at least not in the way we are discussing. So it's all rather academic.
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Last edited by wpoll; 12-29-2018 at 05:16 AM.
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  #128  
Old 12-29-2018, 02:12 AM
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Originally Posted by Fifty150hs View Post
In my experience it doesn't matter. Worn tire vs new it is virtually the same reading on the speedo.
Count yourself lucky then - my experience has been the total opposite.

Esp. on my motorcycle, which has a larger difference between new and worn diameters and wears a lot quicker - 5000-6000 miles from new to worn out!

The math on this is pretty murky, as the actual effective rolling diameter is dependant on tyre brand, sidewall construction, inflation pressure, temperature, vehicle load.... the list goes on. ;-)

Back to the regular scheduled thread subject.... ;-)
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  #129  
Old 12-29-2018, 04:59 AM
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The speedometer is set by bmw to be wrong on all bmw cars. I have a bunch of different models and all are 2 mph at least off. I normally run 57 to do 55 mph. Some are worse off than that.

They do it to cover their butts at home. If a manufacturer there causes a person to get a ticket because of the speedometer they are legally liable.
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  #130  
Old 12-29-2018, 05:01 AM
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Originally Posted by andrewwynn View Post
I'd like to have a Japan gauge to have accurate speed. Of course it would be in kph.
Just code it to be accurate if you want. Its changeable.
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