Reprogramming for winter tires?
 

How do you guys handle the different circumference between the 20" & 18" tires?

The circumference should ideally be the same the aspect or height of the sidewall will change- 215-35-20, 255-50-19 and 255-55-18 should all be within acceptable values, so no need to re-calibrate. not sure look here:
Tire Size Calculator - tire & wheel plus sizing

Agree...if they are oem replacements, and in the zip code size-wise, put them on and drive them.

Unless, the gizmo packed E70 is considerably different/more finicky than the dino E53s...
GL, mD

Those tire size calculators are close, but you should look at the specs for the particular tires you are considering:

go to tirerack.com- look up the model and size you have, and click on the 'specs' tab- look for the overall OD (or the revs per mile)

Try to match those numbers.

The different in revs per mile is precisely the amount the speedo or odo will be affected... a few % is no big deal. (ie 726 versus 740 is a difference of 14; 14/726~2%)

(And this advise only applies when you are changing ALL tires... different advice when mixing sizes...)

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Assuming you haven't done a radical change in overall size, just reinitialise the TPMS system thru the iDrive.

You guys are correct. I looked and the current tires are 726 revs per mile and the snow tires are 718 revs per mile.

Your speedo and odometer will run 8/726=1% slower. NBD

(And my "726" was a total guess!)

Yes, check your air pressures and re-initialize the TPMS

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