Strictly from my opin,
I would think the minor diff in real effect tire size would not be cause
for any dlr to blink and I don't see how that very minor tire size diff
can be an issue for any component or part under warranty.
The PowerDawg Tire Size Calculator shows nominal difference:
Tire size calculatorTire size results
Specification, Sidewall, Radius, Diameter, Circumference, Revolutions Speedometer & Odometer Difference
255/55-18 5.5" 14.5" 29.0" 91.2" 694/mi 60MPH 10000mi N/A
255/60-18 6.0" 15.0" 30.0" 94.4" 671/mi 58MPH 9666mi
3.5%
I can't get the data to paste up properly, but the data is
per the series of terms/categories above.
Your X's speedo is optimistic to begin with, so it should be closer
to true, with the slightly larger tires.
The sizes are really so close, I would not be concerned...
GL,mD
http://www.powerdog.com/tiresize.cgi