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.

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... ;)

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 that’s accurate.

In my experience it doesn't matter. Worn tire vs new it is virtually the same reading on the speedo.

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.

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.... ;-)

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.

Just code it to be accurate if you want. Its changeable.