If you're trying to fit the widest tire I'd guess 245-45-18 without rolling. If it were me I'd want to fit 255-45-18 but you will probably need to roll the fender.
If you like the stretched tire look you can do 225 or 235 with no worries.
Not sure this helps but it's a place to start.
FWIW - Your rim will be approx. 1.25" closer to the fender.
If you're trying to fit the widest tire I'd guess 245-45-18 without rolling. If it were me I'd want to fit 255-45-18 but you will probably need to roll the fender.
If you like the stretched tire look you can do 225 or 235 with no worries.
Not sure this helps but it's a place to start.
FWIW - Your rim will be approx. 1.5" closer to the fender.