I'm looking to change my rear tire sizes to help with under steering and I wanted to know if 235/50R/17 Fronts and 255/45R/17 Rears would be sufficient

Car is a 2003 Nissian 350Z

 

By "help with understeering," do you mean reduce understeer? If you want your car to handle as neutral as possible, get a square setup. The wider the rear tires are in relation to the front, the more the car will understeer. The closer the tire widths are, the more neutral the car will handle. I'm assuming you're currently running the OEM 17s with 225/50 front and 235/50 rear? If so, going with 235/50 front and 255/45 rear will slightly increase understeer.

 

Sorry that's what I meant

Yes it currently has the OEM wheels with 235/50 in the front and rear

 

Just curious, do you track your car a lot to where understeer has become troublesome? If that's the case, you may want to look into adjusting your suspension setup while having as much tire as you can. Sports cars were originally equipped with wider rear tires to induce understeer and keep the cars from spinning out every time the driver guns it at an angle.

 

The quick and simple way to help a lot with understeer on your car, swap to 04.5 and newer rear springs.