Can I use 245/50/17 on my 08 G35 Journey sedan with 225/55/17?

 

It's close. Your sidewall would be slightly smaller as would the diameter and circumference.
Your speedometer would read 0.4% too fast. When your speedo states you are going 60, you would actually be going 59.8 mph.
You could certainly live with that.

Links:
http://www.canadiandriver.com/tools/...c/tirecalc.htm
http://www.jekylhyderacing.com/HeightofTires.htm

 

and assuming you are riding stock height, the wheel gap will be even bigger by that much.

 

Well 235's are 245's to keep the speed close. I wonder how much more I could go with 55's.

 

235/55/17's better then 245/50/17's.