[Pentland]

Originally Posted by inevitab1e

In a 45 I need to drive 54 to be 5 over. It's insane.
The dealer should be able to calibrate these things, IMO

I’m guessing you’re a millennial?
What would have done in the days of mechanical speedo /odo and no GPS? Do the same today and when a pedestrian steps out you probably won’t kill them.

[RockCrusher]

Originally Posted by inevitab1e

I see. So maybe the solution is to go to a thicker tire.
Instead of 225/40/19 up front, go to 225/45/19.
Same with the rear, 255/35/19 to 255/40/19.
That would probably affect the speedo, bigger diameter

Going outside of the tire sizes sanctioned by BMW can bring other problems. Too much difference -- I've read where a 1% circumference difference is the "limit" -- could bring about other problems more annoying and possibly more serious that just a speedo that is a few miles per hour optimistic.

You can visit the dealer and ask if there is some BMW approved way to reduce/eliminate the speedo error.

[inevitab1e]

Originally Posted by RockCrusher

Going outside of the tire sizes sanctioned by BMW can bring other problems. Too much difference -- I've read where a 1% circumference difference is the "limit" -- could bring about other problems more annoying and possibly more serious that just a speedo that is a few miles per hour optimistic.

You can visit the dealer and ask if there is some BMW approved way to reduce/eliminate the speedo error.

A little taller tire front/back would still be the same circumference difference as stock (0.4%). And a smaller circumference than 20" wheels.
I'm sure that would slow the speedo down a tad.

[Pentland]

Originally Posted by RockCrusher

Going outside of the tire sizes sanctioned by BMW can bring other problems. Too much difference -- I've read where a 1% circumference difference is the "limit" -- could bring about other problems more annoying and possibly more serious that just a speedo that is a few miles per hour optimistic.

You can visit the dealer and ask if there is some BMW approved way to reduce/eliminate the speedo error.

That’s not true.
The difference between a new tyre and a worn tyre will be more than 1%. I’d wager even tolerances between manufacturers for the same tyre size are outside 1%.

[RockCrusher]

Originally Posted by Pentland

That’s not true.
The difference between a new tyre and a worn tyre will be more than 1%. I’d wager even tolerances between manufacturers for the same tyre size are outside 1%.

It is possible the difference between a new tire and a worn but otherwise usable tire will be different by more than 1%. In fact I have encountered this several times over the years when I've faced having to replace a (puncture) tire also having to replace the other tire due to too much difference due to wear.

To know for sure what I was told that if one tire needs to be replaced -- due to say a puncture -- the tech will measure the tire on the other end of the axle (and possibly the tires on the other axle) and if the tread depth difference is too great -- how much difference is allowed I'm told depends upon the specific model and the tech has access to this number -- the tech will tell the customer whether just one tire needs to be replaced or if one or possibly all the tires need to be replaced to keep the tire sizes (circumferences) in spec. (The preceding is my experience with at least one other car maker so it is nothing new to me.)

As for differences between tires made by different tire companies...

While tires made by different companies may have differences in diameter if they are made/marked specifically for use on a BMW I would think BMW would require that the tires have the suitably correct dimensions. That is if one bought Brand X tires they would all be within spec. Same for Brand Y.

However, this does not necessarily mean for tires marked for use on BMWs that tires from different tire makers can be mixed.

That is Brand X and Brand Y tires even though both marked specifically for use on BMWs and both nominally have the same "everything" (size, wear rating, etc.) the different brands of tires should not be mixed.

I have encountered this no mixing of tire brands before.

There are the sizes/dimensions but harder to quantify and build to spec is how the tires heat up. Brand X tires and brand Y tires both will have a temperature increase but the two tires from different tires makers may not experience the same increase in temperature under the same conditions.

And tire makers may prohibit mixing of tires and thus BMW would only be following what the tire makers require. In fact one tire maker, Continental, does not sanction the mixing of its tires with other brands of tires.

Bottom line is a BMW tech would be the person to know for sure.

Assuming a 1% maximum allowable difference in circumference this can be just a small fraction of an inch difference in tread depth.

How small of a fraction?

Did the math for my 2024 230xi tires and a 1% difference in circumference works out to an approx. 4/32" tread depth difference.

Here are my notes:

BMW 230xi tires:
Front P225/40 19
Rear: P225/40 19
Dia: 26.1”
Circ: 81.9”
Revs/Mile: 774

26.1” diameter tire has a circumference of 81.995”.

One percent reduction of the new tire circumference:
81.995 - ( 81.995 * 0.01 ) = 81.175”

Based on this new circumference this makes the diameter:
81.175 / pi = 25.838”

And:
26.100” - 25.838” = 0.261”

This works out to a tread depth difference of 0.130” or just over 4/32” (0.125”).

Which is the ballpark difference in tread depth another car maker would require at least both tires on the same axle be replaced.

[inevitab1e]

I may have missed it but wouldn't the trip odometer be wrong as well?
Say you're going 63mph on the dash for an hour, wouldn't the odometer read 63 miles traveled? When in reality it's closer to 60?

[Pentland]

Originally Posted by inevitab1e

I may have missed it but wouldn't the trip odometer be wrong as well?
Say you're going 63mph on the dash for an hour, wouldn't the odometer read 63 miles traveled? When in reality it's closer to 60?

Not necessarily. They can be independent systems and calibrated differently.
For example, you can calculate distance from a rotating wheel of known circumference, the speed of it turning, and the time taken to do so is not required. To calculate speed, you need distance and time, either of which could be off set so as to recalibrate speed.