tire size
 

2005 X5 4.4i, can I use 245 x 60 x 18 instead of 255 x 55 x 18 ? Michelin doesen't make the 255 x 55 in the LTX M/S , I need the LTX for the farm

Your proposed 245/60R18 replacement size, vs the previous size, will
give 6/10ths of an inch more diameter, 1.7" more rolling circumference
and ~ 1 mph speedo difference, reading lower. It is basically a "taller tire".

None of the nominal values suggest it would extremely detrimental, imo.

Handy Dandy tire size calculator:
http://www.powerdog.com/tiresize.cgi

GL, mD

To add to David's post, here are a few additional wheel/tire sizing guides to play around with. (Even though your proposed size will be fine.)

Custom rims, wheel tire packages for your ride - RIMSnTIRES.com
Wheel Offset Calculator
Tire size calculator
Tire Size Calculator - tire & wheel plus sizing