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