Okay so i just got my 350z and i noticed when i drive it and hit a turn while i acelerate the slip light will turn on and my car slows by itself, but i dont completely understand why. One day i looked at my wheels and i noticed that they both 20s (rear: 275/30/20 front: 245/30/20), could that be the main issue?

 

ehhhhh...it could be but probably not...would depend on how fast you are accelerating and how sharp of a steering angle you are pulling.

is your steering wheel / steer angle off-center? needing corrected?

 

If you're activating the slip light, it's more likely you're overdriving the corner, or your tires are just plain worn. If you're going to throttle too aggressively in the turn and your rear tires spin, the traction control system (TCS) will note the difference in rotation from one side to the other. TCS (or VDC for that matter) is designed to cut power to prevent a spin. For a car like the Z, it's a good system, as it keeps drivers from driving their cars off the road.

 

IMHO yes 20s are too big for a 350Z. More rotating mass=poorer performance because of simple physics. More rotating mass dictates a problem with

Velocity, which is a function of change in speed over time (of the wheel and tire). Acceleration and braking become sluggish. IIRC the 350 was introduced on 17s.

 

+1 to all of this ...

both from a physics stand point and a looks standpoing(IMO)...I ran 20s for a while and my car was noticeable slower when I put them on and noticeable faster when I went back to 18" ...

 

I'm with bealljk and jhc, I'm of the view setting-up on a 20" tire/wheel package is off-the-mark from both a performance and aesthetics perspective on a Z. But, rightfully, everyone has their view of what they want when they customize a ride and to each his/her own in pursuing their objectives. If your after performance and ride quality, 20's ain't it.

Also believe dkmura nailed it. If the "nanny" is kicking-in and retarding output a) you're overdriving it, b) your tires are worn, c) something is "broke" in the TCS/VDC system or d) something is "broke/worn" elsewhere (e.g. suspension).

Back to what you intended to ask though, no, your overall diameter/circumference and the delta (ratio) front-to-rear is "not too big" vs OEM specs to cause that. Rollin' on rubber bands, especially if worn, could be a contributor though.

There's more to this than asking "are my wheels too big" and then sharing only tire sizing. Providing the Mfrg and wheel specs (specifically offset & weight) along with tire sizes adds more details. For all we know you have worn 20" rubber on 20" wheels that are ultra-heavy, etc. (that "physics" thing mentioned above is more of a factor with 20's vs 17" - 19" wheel-sets).