Quote:
Originally Posted by EnerJi
Just be warned, assuming the diameter of your new tires is roughly equal to the difference between the 18" and 20" wheels (1/9 larger), then your speed (and the trip meter and odometer) will be approximately 90% of the true value. Your speed will show 65 when in actuality you're going about 72.
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Outside diameter remains very close to the same when properly upsizing...larger wheel means lower profile tire. When going from the OEM 18s to the OEM 20s, the difference in outside diameter of tires is miniscule and the difference in speedo readout is practically nonexistent. Not sure if the speed is measured from the rotation of the front or rear tires, but see the attached file to see the speedo differences for front and rear. (That's not to say that some of the configurations that I have seen on here won't make a difference as I have seen several setups here where the outside diameter is smaller/larger by a more significant amount based on the tires sizes people are using)
To the OP, I'm pretty sure your dealer/service advisor is just referring to the TPMS reset, which as ric124 notes is very easily done yourself.