Right now I have 18x8.5 SSR comps, I am looking to change my tires soon, so I was wondering if I went with 245/35/18 would it be same is going down to 17 inch wheels with 45 or 40 series tires? I figure I can get some gearing benefits from getting smaller diameter tires, good idea?

 

a 245/35/18 is 24.8" tall (give or take 2/10s based on which tire you use) while a 245/40/17 is 24.7" tall.

I dont think that i would recomend it for gearing benefits at all. Your speedo wont read correctly if you go with to small or too big a tire and as a result you wont have an accurate mileage count on your car.
just my .02

 

The first post in the first sticky in this sub-forum has some very good links on the subject. LINK But the quick answer is on the track YES, but day to day driving not that noticeable.