you're correct, there is no standard for UTQG numbers. Many make the mistake of using these numbers to cross shop tires. A 300 rating on one brand, may not have same wear equivalency as a 300 rating on another brand. You can use the numbers as a relative rating within the same brand line but thats about it.

 

i run the s drives on my coupe as well...great tires and great handling!!

 

that is fine. The second number in the size is the size of ur sidewall. That number is a percent of the width of your tire. by using 245/40 in the back and 225/45 in the front you bring the size of the sidewall closer to the same size. But in that case you are only talking a few mm anyway.
your stock 18s are 8.5 all the way around, the factory just puts a smaller tire on the front. So i recommend 245 square set up so you can rotate them and get more life out of them. also you can fit up to a 275 in the rear... youll have a nice meaty fitment but it can be done. also tirerack.com is a great place with a ton of info about the ratings of tires, what all the numbers and letters mean, speed ratings etc...
in the end my recommendation, if your looking for it, is 245/45/18 all the way round

 

your right it makes sense to put 245's all around but my front ones still have quiet a bit of life on them so i am just going to swap the back ones for now and when the time for fronts come i'll put those in.

so no on the 255/35/18? and yes on the 245/40/18

 

not that you cant run 255s i just wouldnt bc you cant rotate them when the time comes. id buy 245s put them on the rear, then when the fronts go, move the backs to the front then put ur new 245s on the rear and rotate them accordingly... or buy the cheapest tires in any size that fits, roast the pi$$ out of all of them, have some fun and start over with four new tires in the spring

 

I like this idea lol

 

ok so finalizing...need some +1's from you experienced guys...

leave the 225/45/18's alone in the front

get 2 245/40/18's in the rear

got couple options:

i am thinking either the michelin or the goodyear

(2)CONTINENTAL 245/40/ZR18 SPORT CONTACT 2 65%TREAD $100 DOLLARS 280 AA A link

(2)CONTINENTAL 245/40/18 CONTI PRO CONTACT 70%TREAD $100 DOLLARS 400 AA A link

(2)FALKIN 245/40/18 ZIEX 912 65%TREAD $80 DOLLARS

(2)GOODYEAR 245/40/zr18 EGALE GT 70%TREAD $100 DOLLARS 400 AA A link

(2)MICHELIN 245/40/18 65%TREAD PILOT HX MXM4 $80 DOLLARS 300 A A link

edit: after finding more information i think the bold one is my best option?

 

You need to keep a staggered setup with the G35. I would not go 40 in the rear with 45 in the front. At the least, stick with 45 in the rear.