I want to go a bit wider tires on the rear. My friends recommended 255/40/18 rather than 255/45/18 because the thicker sidewall would increase cornering instability. Is that true?

 

just cuz the tire is wider in size does not mean the sidewall will be wider.. the wider tires should help cuz you ll have more rubber resulting in more traction but how much will 10mm help and other tire brands could also help as well.. i would do it just cuz i like meat on my wheels

 

Theortically, an increase in tire wall aspect ratio (or you can just call it height) will give a softer sidewall (assuming that you are comparing the same brand/type/model of tire), which is known to provide less conering stability.