Hey guys will a 245/45 18 ft on the back of the c55?

I dont think you should run a 45 with 18 in the back,

if you want to run 18, run 255 or 265 40/18.

because on the 17s, its 245/45/17.

Ya i know i really shouldnt run these tires but i have 2 brand new ones just laying around and wanted to use them if possible. They are Michelin plilot sports. 245/45/18


Anyone know if i'll encounter any rubbing issues or any other fitment problems?

no you can't

going from a 245/35/18 (stock tyre size) to a 245/45/18 will increase your sidewall by too much and throw your speedo out + there's a chance it may rub on your arches.

go here and punch in your sizes to see for yourself

http://www.alloywheels.com/tyrecalc.asp

Might be a bit too tall. I used 245/40R18 on the rear of my C55 because that was the best fit with RT-615. But, the 45 series is 10% taller so I'm not sure that will work.