i am probably gonna be getting new tires in the coming month or so, definitely by the end of summer.

I am currently running stock 225-45

i like the "lightness" and the response i am getting from the lighter tire on autox.

question is, should i up the tire to 245-40?

grip should increase slightly, but i don't know, does the mpg and "lightness" of the car decrease?

 

b-stock champ 3 years running, runs 245/40.

toyo t1r is a lightweight tire. it may even be lighter in 245/40 than the potenza in 225/45. but i cant confirm that.

 

Nope. B Stock champ three years running runs 245/35-18 V710s. Not that it matters, since R comps to street tires is apples to oranges anyway.

All things equal, a 245/40-18 should be slightly shorter than a 225/45-18, so acceleration should improve, but not by any amount that you'll be able to notice.

"Lightness" or weight of tire will vary form manufacturer to manufacturer and model to model. In other words, you could pick a 245/40-18 that's lighter than the OEM tires, or you could pick one that is several pounds heavier. Look at somewhere like Tire Rack for weight on specific tire sizes.

 

i just checked, the stock bridgestone is 28lbs, the toyo t1r is 26lbs in 240/40/18

 

Responsiveness is not just an issue of the lightness of the tire, but also that of sidewall support and stiffness. The T1R might not be as responsive as its light weight implies since it also has a reputation for having a soft sidewall, and a 245/40 on an 8" wheel lacks the wheel support that a 225/45 has on the same width.